Rename model 'gemini-3.1-flash-lite-preview' to 'gemini-3.1-flash-lite'

[discountry]

Pull Request Description

Summary

This PR removes the deprecated Gemini 3.1 Flash Lite Preview model, which was officially discontinued on May 25, 2026. The codebase has been updated to use the active production model name.

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Changes

• Model Deprecation: Removed references and configuration blocks for Gemini 3.1 Flash Lite Preview.
• Model Rename: Updated the underlying API identifiers and configuration variables to point to the correct, non-preview model name.

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Verification

• Ran unit tests to ensure no stale model references remain.
• Verified that API calls successfully fall back to or directly call the newly designated model name.

[gemini-code-assist]

Code Review

This pull request updates the model registry definitions in models.json to rename the gemini-3.1-flash-lite-preview model to its stable release name, gemini-3.1-flash-lite, across multiple entries. This includes updating the id, display_name, and name fields. There are no review comments, and I have no additional feedback to provide.