feat: 添加 Exa AI 搜索适配器

- 新增 ExaSearchAdapter，基于 MCP 协议调用 Exa 搜索 API
- WebSearchTool 支持 num_results、livecrawl、search_type、context_max_characters 等高级选项
- 非 Anthropic 官方 base URL 时默认使用 Exa 适配器

packages/builtin-tools/src/tools/WebSearchTool/WebSearchTool.ts

@@ -23,6 +23,26 @@ const inputSchema = lazySchema(() =>
       .array(z.string())
       .optional()
       .describe('Never include search results from these domains'),
+    num_results: z
+      .number()
+      .optional()
+      .describe('Number of search results to return (default: 8)'),
+    livecrawl: z
+      .enum(['fallback', 'preferred'])
+      .optional()
+      .describe(
+        "Live crawl mode - 'fallback': use live crawling as backup if cached content unavailable, 'preferred': prioritize live crawling (default: 'fallback')",
+      ),
+    search_type: z
+      .enum(['auto', 'fast', 'deep'])
+      .optional()
+      .describe(
+        "Search type - 'auto': balanced search (default), 'fast': quick results, 'deep': comprehensive search",
+      ),
+    context_max_characters: z
+      .number()
+      .optional()
+      .describe('Maximum characters for context string optimized for LLMs (default: 10000)'),
   }),
 )
 type InputSchema = ReturnType<typeof inputSchema>
@@ -148,6 +168,10 @@ export const WebSearchTool = buildTool({
     const adapterResults = await adapter.search(query, {
       allowedDomains: input.allowed_domains,
       blockedDomains: input.blocked_domains,
+      numResults: input.num_results,
+      livecrawl: input.livecrawl,
+      searchType: input.search_type,
+      contextMaxCharacters: input.context_max_characters,
       signal: context.abortController.signal,
       onProgress(progress) {
         if (onProgress) {