feat(tools): add Bocha web search provider

[shikihane]

Description

Add Bocha AI as a new web search provider, appended at the lowest priority in the search provider chain.

Changes:

• Config: Add BochaConfig struct and Bocha field in WebToolsConfig
• Web tools: Implement BochaSearchProvider using the Bocha AI web search API (api.bochaai.com/v1/web-search), supporting configurable API key, base URL, and max results
• Agent loop: Wire Bocha config fields into WebSearchToolOptions

Provider priority remains: Perplexity > Brave > Tavily > DuckDuckGo > Bocha

Type of Change

• New feature (non-breaking change which adds functionality)

AI Code Generation

• Mostly AI-generated code with human review and testing

Technical Context

Bocha AI provides a Bing-compatible web search API with optional summary generation. The implementation follows the existing SearchProvider interface pattern. Response format: data.webPages.value[] with name, url, snippet, summary fields. Summaries are preferred over snippets when available, truncated at 500 chars.

Test Environment

• Radxa Cubie A7A (arm64), Debian 11
• Verified Bocha API with curl and live queries
• Build and all tests pass

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• New and existing tests pass locally with my changes
• Any dependent changes have been already merged and published

🤖 Generated with Claude Code

[nikolasdehor]

Clean implementation that follows the existing SearchProvider pattern well. Priority placement (lowest, below DuckDuckGo) makes sense. A few suggestions: (1) Unbounded response read: io.ReadAll(resp.Body) has no size limit. Consider io.LimitReader(resp.Body, 110241024) to prevent OOM from malformed responses (PR #883 addresses the same pattern in WebFetch). (2) HTTP client per-request: createHTTPClient is called on every Search() invocation, creating a new client and transport each time. Consider creating it once in the constructor for HTTP connection reuse. (3) Tests: Even a basic test with a mock HTTP server would be valuable. LGTM.