fix: web_search 폴백 체인이 빈 결과를 성공으로 오판하던 버그 수정

executeWebSearch()가 provider 호출이 예외를 던지지 않으면 결과가
0건이어도 "성공"으로 간주하고 그 자리에서 반환해버려, 뒤에 있는
(더 잘 작동할 수도 있는) provider로 넘어가지 못했음. 오늘 시어엔진이
bing 타임아웃으로 0건을 반환했을 때 ollama_cloud가 멀쩡히 있었는데도
전혀 시도되지 않은 게 이 버그 때문이었음.

- SearchProviderAttempt에 'empty' 상태 추가 (성공했지만 결과 없음)
- 각 provider 호출 후 실제 결과 개수를 확인해서 0건이면 다음
  provider로 계속 진행하도록 수정
- 전부 empty/failed로 끝나면 하드 에러 대신 가장 처음 나온 "성공했지만
  빈 결과"를 우아하게 반환 (기존엔 여기까지 오면 무조건 에러)
- ddg를 candidates 맨 뒤로 재배치 — Instant Answer API는 구조적으로
  일반 검색을 지원 안 하고, HTML 스크래핑 폴백은 현재 이 서버 IP에서
  DuckDuckGo 봇탐지에 막혀있어 거의 항상 빈손임 (html.duckduckgo.com,
  lite.duckduckgo.com, duckduckgo.com 전부 확인함)
- 중복되던 6개의 거의 동일한 provider 분기를 runProvider() 디스패치
  헬퍼 하나로 통합

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/tools/web.ts

@@ -26,7 +26,7 @@ type StructuredFact = { id: number; claim: string; evidence_ids: number[]; sourc
 type SearchProvider = 'tavily' | 'google' | 'brave' | 'ddg' | 'ddg_html' | 'searxng' | 'ollama_cloud';
 type SearchProviderAttempt = {
   provider: SearchProvider;
-  status: 'success' | 'failed' | 'skipped';
+  status: 'success' | 'empty' | 'failed' | 'skipped';
   reason?: string;
   duration_ms?: number;
   result_count?: number;
@@ -813,7 +813,10 @@ export async function executeWebSearch(args: { query: string; max_results?: numb
 
   const cfg = getSearchConfig();
 
-  const candidates: Array<'tavily' | 'searxng' | 'google' | 'brave' | 'ddg' | 'ollama_cloud'> = ['searxng', 'ollama_cloud', 'ddg', 'tavily', 'google', 'brave'];
+  // ddg sits last: its Instant Answer API only covers infobox-style queries and its
+  // HTML scrape fallback is routinely bot-walled (see searchDDGHtml) — it rarely adds
+  // value once searxng/ollama_cloud/tavily/google/brave have all had a shot.
+  const candidates: Array<'tavily' | 'searxng' | 'google' | 'brave' | 'ollama_cloud' | 'ddg'> = ['searxng', 'ollama_cloud', 'tavily', 'google', 'brave', 'ddg'];
   const providerOrder = [cfg.preferred, ...candidates.filter(p => p !== cfg.preferred)];
   const diagnostics: SearchDiagnostics = {
     query: args.query,
@@ -822,7 +825,24 @@ export async function executeWebSearch(args: { query: string; max_results?: numb
     attempted: [],
   };
 
+  const runProvider = (provider: SearchProvider): Promise<ToolResult> => {
+    switch (provider) {
+      case 'tavily':       return searchTavily(args.query, limit, cfg.tavilyKey as string);
+      case 'searxng':      return searchSearXNG(args.query, limit, cfg.searxngUrl as string);
+      case 'google':       return searchGoogle(args.query, limit, cfg.googleKey as string, cfg.googleCx as string);
+      case 'brave':        return searchBrave(args.query, limit, cfg.braveKey as string);
+      case 'ollama_cloud': return searchOllamaCloud(args.query, limit, cfg.ollamaApiKey as string);
+      case 'ddg':           return searchDDG(args.query, limit);
+      default: throw new Error(`unhandled provider: ${provider}`);
+    }
+  };
+
   let lastErr = null;
+  // A provider that returned HTTP-OK but zero results is kept as a last-resort answer —
+  // better than a hard error if every provider ends up empty for this query.
+  let bestEmpty: ToolResult | null = null;
+  let bestEmptyProvider: SearchProvider | null = null;
+
   for (const provider of providerOrder) {
     if (provider === 'tavily' && !cfg.tavilyKey) {
       diagnostics.attempted.push({ provider, status: 'skipped', reason: 'missing_tavily_api_key' });
@@ -847,89 +867,27 @@ export async function executeWebSearch(args: { query: string; max_results?: numb
 
     const started = Date.now();
     try {
-      if (provider === 'tavily') {
-        const res = await searchTavily(args.query, limit, cfg.tavilyKey as string);
-        await augmentEventContract(args.query, res);
-        const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
-        diagnostics.attempted.push({
-          provider,
-          status: 'success',
-          duration_ms: Date.now() - started,
-          result_count: resultCount,
-        });
-        diagnostics.selected_provider = 'tavily';
-        res.data = { ...(res.data || {}), provider: 'tavily', search_diagnostics: diagnostics };
+      const res = await runProvider(provider);
+      await augmentEventContract(args.query, res);
+      const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
+      const hasResults = res.success && resultCount > 0;
+
+      diagnostics.attempted.push({
+        provider,
+        status: hasResults ? 'success' : (res.success ? 'empty' : 'failed'),
+        duration_ms: Date.now() - started,
+        result_count: resultCount,
+        ...(!res.success && { reason: res.error }),
+      });
+
+      if (hasResults) {
+        diagnostics.selected_provider = provider;
+        res.data = { ...(res.data || {}), provider, search_diagnostics: diagnostics };
         return res;
       }
-      if (provider === 'searxng') {
-        const res = await searchSearXNG(args.query, limit, cfg.searxngUrl as string);
-        await augmentEventContract(args.query, res);
-        const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
-        diagnostics.attempted.push({
-          provider,
-          status: 'success',
-          duration_ms: Date.now() - started,
-          result_count: resultCount,
-        });
-        diagnostics.selected_provider = 'searxng';
-        res.data = { ...(res.data || {}), provider: 'searxng', search_diagnostics: diagnostics };
-        return res;
-      }
-      if (provider === 'google') {
-        const res = await searchGoogle(args.query, limit, cfg.googleKey as string, cfg.googleCx as string);
-        await augmentEventContract(args.query, res);
-        const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
-        diagnostics.attempted.push({
-          provider,
-          status: 'success',
-          duration_ms: Date.now() - started,
-          result_count: resultCount,
-        });
-        diagnostics.selected_provider = 'google';
-        res.data = { ...(res.data || {}), provider: 'google', search_diagnostics: diagnostics };
-        return res;
-      }
-      if (provider === 'brave') {
-        const res = await searchBrave(args.query, limit, cfg.braveKey as string);
-        await augmentEventContract(args.query, res);
-        const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
-        diagnostics.attempted.push({
-          provider,
-          status: 'success',
-          duration_ms: Date.now() - started,
-          result_count: resultCount,
-        });
-        diagnostics.selected_provider = 'brave';
-        res.data = { ...(res.data || {}), provider: 'brave', search_diagnostics: diagnostics };
-        return res;
-      }
-      if (provider === 'ddg') {
-        const res = await searchDDG(args.query, limit);
-        await augmentEventContract(args.query, res);
-        const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
-        diagnostics.attempted.push({
-          provider,
-          status: 'success',
-          duration_ms: Date.now() - started,
-          result_count: resultCount,
-        });
-        diagnostics.selected_provider = 'ddg';
-        res.data = { ...(res.data || {}), provider: 'ddg', search_diagnostics: diagnostics };
-        return res;
-      }
-      if (provider === 'ollama_cloud') {
-        const res = await searchOllamaCloud(args.query, limit, cfg.ollamaApiKey as string);
-        await augmentEventContract(args.query, res);
-        const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
-        diagnostics.attempted.push({
-          provider,
-          status: 'success',
-          duration_ms: Date.now() - started,
-          result_count: resultCount,
-        });
-        diagnostics.selected_provider = 'ollama_cloud';
-        res.data = { ...(res.data || {}), provider: 'ollama_cloud', search_diagnostics: diagnostics };
-        return res;
+      if (res.success && !bestEmpty) {
+        bestEmpty = res;
+        bestEmptyProvider = provider;
       }
     } catch (err) {
       lastErr = err;
@@ -942,20 +900,28 @@ export async function executeWebSearch(args: { query: string; max_results?: numb
     }
   }
 
-  // Final fallback if ddg path threw and wasn't already successful
+  // Final fallback: DDG HTML scrape (rarely succeeds — see comment on `candidates` above)
   const fallbackStarted = Date.now();
   try {
     const res = await searchDDGHtml(args.query, limit);
     const resultCount = Array.isArray(res.data?.results) ? res.data.results.length : 0;
+    const hasResults = res.success && resultCount > 0;
     diagnostics.attempted.push({
       provider: 'ddg_html',
-      status: 'success',
+      status: hasResults ? 'success' : (res.success ? 'empty' : 'failed'),
       duration_ms: Date.now() - fallbackStarted,
       result_count: resultCount,
+      ...(!res.success && { reason: res.error }),
     });
-    diagnostics.selected_provider = 'ddg_html';
-    res.data = { ...(res.data || {}), provider: 'ddg_html', search_diagnostics: diagnostics };
-    return res;
+    if (hasResults) {
+      diagnostics.selected_provider = 'ddg_html';
+      res.data = { ...(res.data || {}), provider: 'ddg_html', search_diagnostics: diagnostics };
+      return res;
+    }
+    if (res.success && !bestEmpty) {
+      bestEmpty = res;
+      bestEmptyProvider = 'ddg_html';
+    }
   } catch (err) {
     lastErr = err;
     diagnostics.attempted.push({
@@ -965,6 +931,13 @@ export async function executeWebSearch(args: { query: string; max_results?: numb
       duration_ms: Date.now() - fallbackStarted,
     });
   }
+
+  if (bestEmpty) {
+    diagnostics.selected_provider = bestEmptyProvider as SearchProvider;
+    bestEmpty.data = { ...(bestEmpty.data || {}), provider: bestEmptyProvider, search_diagnostics: diagnostics };
+    return bestEmpty;
+  }
+
   let errMsg = 'unknown error';
   if (lastErr) {
     if (typeof lastErr === 'object' && 'message' in lastErr) errMsg = (lastErr as any).message;