feat: update Gemini and Vertex AI models from preview to GA version

[roomote]

Fixes #5444

This PR updates the Gemini and Vertex AI model naming conventions from outdated preview versions to the GA (Generally Available) version, as requested in issue #5444.

Changes Made

Model Definition Updates

• Removed outdated preview models from type definitions:
  • gemini-2.5-pro-preview-03-25
  • gemini-2.5-pro-preview-05-06
  • gemini-2.5-pro-preview-06-05
• Updated to GA model: gemini-2.5-pro across all providers

Backward Compatibility Implementation

• Added legacy model detection in Gemini provider with isLegacyPreviewModel() method
• Implemented automatic mapping from preview models to GA version
• Extended Vertex provider to inherit the same backward compatibility logic
• Added OpenRouter preview model mapping for google/gemini-2.5-pro-preview

Test Updates

• Fixed OpenRouter test expectations to reflect removal of preview model references
• Updated test endpoints to use GA model instead of preview

Testing

• All existing tests should pass with backward compatibility
• Users with existing preview model selections will automatically get mapped to the GA version
• No breaking changes for existing configurations

Verification of Acceptance Criteria

• Remove outdated preview models: All preview model definitions removed from type files
• Update to GA version: gemini-2.5-pro is now the current model across all providers
• Maintain backward compatibility: Users with existing preview model selections will automatically get mapped to the GA version
• Update related integrations: OpenRouter provider updated accordingly

Impact

This change ensures that:

1. Users get the most current and supported model from Google
2. Existing configurations continue to work without manual intervention
3. Future model updates follow Google's official naming conventions
4. No service disruption for current users with preview model selections

---

Important

Update Gemini and Vertex AI models from preview to GA version, ensuring backward compatibility and updating tests.

• Model Definition Updates:
  • Removed preview models gemini-2.5-pro-preview-03-25, gemini-2.5-pro-preview-05-06, gemini-2.5-pro-preview-06-05 from gemini.ts and vertex.ts.
  • Updated to GA model gemini-2.5-pro in gemini.ts and vertex.ts.
• Backward Compatibility:
  • Added isLegacyPreviewModel() in GeminiHandler and OpenRouterHandler to map preview models to GA version.
  • VertexHandler inherits backward compatibility logic from GeminiHandler.
• Test Updates:
  • Updated openrouter.spec.ts to reflect removal of preview models and use GA model google/gemini-2.5-pro.

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[daniel-lxs]

Closing in favor of #5990