fix(google): allow opting out of the direct Gemini -tiered wire rename

[chilung-cgu]

Summary

Google's Generative Language API renamed current Gemini Flash generations on the wire with a -tiered suffix (gemini-3.7-flash -> gemini-3.7-flash-tiered, gemini-3.6-flash -> gemini-3.6-flash-tiered), and opencodex applies that rename by default in the AI Studio (direct) path. Some AI Studio deployments/keys still serve the bare ids and return 404 NOT_FOUND (models/gemini-3.7-flash-tiered is not found for API version v1beta) for every renamed request.

This PR adds a per-provider opt-out: directGeminiWireRenames: false keeps the picker-visible id stable while sending the bare id on the wire. Default behavior (rename applied) is unchanged; Vertex requests are unaffected.

Verification

• bun run typecheck passes.
• bun test tests/google-adapter.test.ts tests/config.test.ts passes (159 tests, including new coverage for the opt-out round-trip and the Vertex exclusion).
• Live check against generativelanguage.googleapis.com/v1beta/models confirmed the affected account serves bare ids only; with the opt-out set, the request reaches models/gemini-3.7-flash:generateContent and succeeds.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed (providers.md documents the new setting).
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

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Review readiness checklist

• ⬜ All CI tests are green on my local testing.
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• ⬜ I resolved all correct Codex and CodeRabbit findings.
• ⬜ My PR is ready for review.

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[chilung-cgu]

Duplicate of #1739 (same fix: directGeminiWireRenames opt-out, cherry-picked onto latest dev). Keeping #1739; closing this one to avoid double review. Reopen if preferred.