feat: add BoCha web search provider (#700)

* feat: add BoCha web search provider

Add BoCha (bochaai.com) as a new web search provider option alongside
Bing and Tavily, with API key configuration and connection check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address review feedback for BoCha web search provider

- Fix response access path: ofetch returns parsed JSON directly, not wrapped in .data
- Register bocha_api_key_required in ChatboxAIAPIError.codeNameMap (was throwing null)
- Replace `as any` with proper union type for provider setting
- Add snippet fallback since BoCha API summary field is optional

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
(cherry picked from commit bc7727251fe04e04ff974c2fa2ba6c0aea9a281b)