fix: pass through custom model names in Gemini provider

[roomote-v0]

Related GitHub Issue

Closes: #12136

Description

This PR attempts to address Issue #12136. Feedback and guidance are welcome.

Root cause: In src/api/providers/gemini.ts, the getModel() method checked if the user-provided model ID existed in the predefined geminiModels map. If not found, it silently fell back to geminiDefaultModelId (gemini-3.1-pro) instead of passing through the custom model name.

Fix: Updated getModel() to handle three cases (matching the pattern already applied to vertex.ts in commit 9724a28 for #12074):

1. If modelId is in geminiModels: use the curated ModelInfo (existing behavior).
2. If modelId is provided but not in the map: pass it through as-is with sensible default ModelInfo (new -- fixes the bug).
3. If no modelId is provided: fall back to geminiDefaultModelId (existing behavior).

Test Procedure

• Updated existing test in gemini.spec.ts that previously asserted custom models fall back to the default. It now verifies custom model names are passed through as-is.
• Added a new test verifying the default model is still used when no model ID is specified.
• All 20 tests in gemini.spec.ts pass. All 17 tests in vertex.spec.ts and gemini-handler.spec.ts continue to pass.
• Run: cd src && npx vitest run api/providers/__tests__/gemini.spec.ts

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue.
• Scope: Changes are focused on the linked issue.
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and updated tests have been added to cover changes.
• Documentation Impact: No documentation updates are required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

This is structurally identical to the fix already applied for Vertex AI in #12074 (commit 9724a28). The same silent-fallback pattern existed in the Gemini provider and is now resolved with the same approach.

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