fix(zen): strip auth header for zen google requests

[abeisleem]

Issue for this PR

Closes #35338

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Zen's public OpenAI-compatible chat/completions route authenticates callers with Authorization: Bearer <OPENCODE_API_KEY>, but Google-backed upstream requests should use x-goog-api-key only.

For Gemini-backed Zen models, the proxy was forwarding the incoming Authorization header upstream in addition to setting x-goog-api-key. GCP then treats that bearer token as the upstream auth credential and rejects the request with ACCESS_TOKEN_TYPE_UNSUPPORTED / UNAUTHENTICATED.

This PR strips the forwarded Authorization header in the Google helper before setting x-goog-api-key, and adds a focused regression test for that header behavior.

Relevant Gemini REST docs:

• https://ai.google.dev/gemini-api/docs/api-key
• https://ai.google.dev/gemini-api/docs/text-generation

Both docs show API-key-authenticated REST requests using the x-goog-api-key header.

How did you verify your code works?

• reproduced the public API failure manually against https://opencode.ai/zen/v1/chat/completions with model: gemini-3-flash
• confirmed the Google-native Zen endpoint works when the request uses only x-goog-api-key
• added a focused unit regression test for the Google helper header rewrite
• ran:
  • bun test test/providerUsage.test.ts test/googleProvider.test.ts
  • bun typecheck

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Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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