#13366: Add baidu search as web tools provider

<!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a new `baidu` option to the `web_search` tool, including configuration types/schema updates and a new Baidu Search API request path in `src/agents/tools/web-search.ts`. Docs are updated to mention Baidu as a provider and include setup examples. The integration follows the same provider-selection pattern as Brave/Perplexity/Grok, but the Baidu response payload currently diverges from existing web_search conventions around marking and wrapping untrusted external content, and the Baidu API key is not treated as a sensitive field in the runtime config schema. <h3>Confidence Score: 3/5</h3> - This PR is close to mergeable but has a couple of security/consistency issues to address first. - Core provider wiring and schema/type updates look coherent, but Baidu results omit the existing `externalContent` untrusted marker convention and the Baidu API key is not marked sensitive in the runtime schema, which can lead to unsafe handling and accidental secret exposure. - src/agents/tools/web-search.ts, src/config/zod-schema.agent-runtime.ts _{Last reviewed commit: ab2344f} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->