fix: WebSearch providers + MCPTool bugs (#593)

* fix: WebSearch providers + MCPTool bugs

WebSearchTool:
- custom.ts: fix buildAuthHeadersForPreset WEB_AUTH_HEADER opt-out
- custom.ts: fix WEB_AUTH_SCHEME empty string handling
- custom.ts: fix walkJsonPath null safety for jsonPath parsing
- duckduckgo.ts: use SafeSearchType enum instead of raw 0
- mojeek.ts: always send Accept: application/json header
- README: fix timeout documentation (15s -> 120s to match code)
- custom.test.ts: add tests for auth header behavior

MCPTool:
- MCPTool.ts: fix outputSchema to accept ContentBlockParam[] (not just string)
- MCPTool.ts: fix isResultTruncated for array output (iterates text blocks)

* fix: address PR #593 review feedback

1. Export buildAuthHeadersForPreset and add direct tests for:
   - WEB_AUTH_HEADER="" explicit opt-out behavior
   - WEB_AUTH_SCHEME="" stripping scheme prefix
   - Preset defaults (authHeader + authScheme)
   - No WEB_KEY returns empty headers

2. Add duckduckgo.test.ts verifying SafeSearchType.STRICT === 0,
   confirming the enum change is semantically identical to the
   previous raw value.

Addresses review by @Vasanthdev2004 at
pullrequestreview-4093533095

---------

Co-authored-by: FluxLuFFy <flux@openclaude.dev>
Co-authored-by: Fix Bot <fix@openclaude.local>

src/tools/WebSearchTool/providers/duckduckgo.ts

@@ -12,14 +12,15 @@ export const duckduckgoProvider: SearchProvider = {
   async search(input: SearchInput, signal?: AbortSignal): Promise<ProviderOutput> {
     const start = performance.now()
     let search: typeof import('duck-duck-scrape').search
+    let SafeSearchType: typeof import('duck-duck-scrape').SafeSearchType
     try {
-      ;({ search } = await import('duck-duck-scrape'))
+      ;({ search, SafeSearchType } = await import('duck-duck-scrape'))
     } catch {
       throw new Error('duck-duck-scrape package not installed. Run: npm install duck-duck-scrape')
     }
     if (signal?.aborted) throw new DOMException('Aborted', 'AbortError')
     // TODO: duck-duck-scrape doesn't accept AbortSignal — can't cancel in-flight searches
-    const response = await search(input.query, { safeSearch: 0 })
+    const response = await search(input.query, { safeSearch: SafeSearchType.STRICT })
 
     const hits = applyDomainFilters(
       response.results.map(r => ({