[codex] Add Bocha web search provider

[NiTingKY]

Summary

• Add Bocha as a built-in web search provider for AingDesk's existing online search flow.
• Expose Bocha in the default search engine selector.
• Document the runtime environment variables needed for Bocha search.
• Add focused tests for Bocha response mapping and provider registration.

Validation

• npx.cmd vitest run electron/search_engines/bocha.test.ts -> 2 tests passed
• npm.cmd run build-electron -> exit 0, with the existing ES2024 target warning from tsconfig.json
• Real provider smoke test with BOCHA_SEARCH_API_KEY and BOCHA_SEARCH_RESULT_COUNT=3 -> returned 3 results

Notes

The Bocha API key is read from BOCHA_SEARCH_API_KEY at runtime and is not committed.