feat: add Gemini 3.1 Flash Lite Preview model

[roomote-v0]

Related GitHub Issue

Closes: #11872

Description

This PR attempts to address Issue #11872 by adding the gemini-3.1-flash-lite-preview model to both the Gemini and Vertex AI providers.

The model is referenced in the Google AI docs and is described as a text-out model.

Changes:

• Added gemini-3.1-flash-lite-preview to packages/types/src/providers/gemini.ts
• Added gemini-3.1-flash-lite-preview to packages/types/src/providers/vertex.ts

Model configuration is based on the existing flash lite tier pricing pattern (inputPrice: 0.1, outputPrice: 0.4) and the 3.x generation conventions. Since Google describes this as a "Text-out" model, supportsImages is set to false. Pricing values may need adjustment once official pricing is confirmed.

Feedback and guidance are welcome.

Test Procedure

• Ran existing Gemini provider tests: all 19 tests pass
• Ran existing Vertex provider tests: all 7 tests pass
• Lint and type checks pass across the entire monorepo

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

• The supportsImages: false flag is set based on the "Text-out" model designation. If the model actually supports image inputs, this should be updated.
• Pricing is estimated based on the existing 2.5 Flash Lite models. Official pricing should be verified when available.

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