[codex] fix(gemini): pass custom http options to sdk

[yzlu0917]

Summary

• pass requestOptions.headers and requestOptions.timeout into the GoogleGenAI SDK via httpOptions
• forward custom Gemini apiBase values into the SDK so chat/stream requests use the same endpoint override as embed requests
• add adapter tests covering custom headers, custom base URL forwarding, and the default no-httpOptions path

Why

Continue's Gemini adapter already honored custom request options for the manual embed() path, but chat and streaming requests were created through GoogleGenAI without any of the configured requestOptions.headers or custom apiBase. That meant API gateways expecting custom auth headers like x-api-key could work for one path and silently fail for the main chat path.

This change keeps the SDK-based Gemini implementation, but makes its HTTP configuration consistent with the rest of the adapter.

Validation

• ran npm test -- src/test/gemini-adapter.vitest.ts src/test/main.test.ts in packages/openai-adapters

Closes #12008

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Summary by cubic

Fixes the Gemini adapter to pass custom headers and timeouts to the @google/genai SDK and to forward a custom apiBase for chat and streaming. This makes gateway-authenticated requests work consistently across embed, chat, and stream.

• Bug Fixes
  • Build httpOptions from requestOptions.headers and requestOptions.timeout and pass to GoogleGenAI.
  • Forward custom apiBase to the SDK (clears apiVersion) so chat/stream use the same endpoint as embeds.
  • Added tests for custom headers, base URL forwarding, and the default path without httpOptions.

^{Written for commit daca94d. Summary will update on new commits.}

[cubic-dev-ai]

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