feat(web): add Metaso as a web search provider (metaso.cn) (#2059)

Adds Metaso AI Search as a new SearchProvider option alongside Bing,
DuckDuckGo, Tavily, and Bocha.

Co-authored-by: Zhao Xiaohong <zhaoxiaohong@metasota.ai>

config.example.toml

@@ -271,11 +271,13 @@ max_subagents = 10 # optional (1-20)
 # Switch to Tavily or Bocha for reliable search in mainland China.
 #
 # [search]
-# provider = "bing"         # bing | duckduckgo | tavily | bocha
+# provider = "bing"         # bing | duckduckgo | tavily | bocha | metaso
 #                            # duckduckgo: HTML scrape with Bing fallback
 #                            # tavily: https://tavily.com — AI search, needs api_key
 #                            # bocha:  https://bochaai.com — 博查AI搜索，国内友好，需api_key
-# api_key = "tvly-YOUR_KEY"  # required for tavily and bocha
+#                            # metaso: https://metaso.cn — 秘塔AI搜索，每天 100 次免费
+#                            #         设置 METASO_API_KEY 或 [search] api_key 可提升额度
+# api_key = "tvly-YOUR_KEY"  # required for tavily, bocha, and metaso (optional for metaso)
 #                            # WARNING: treat config.toml like a secret file when
 #                            # storing API keys. Use env vars or `auth set` instead.
 #