fix(core): add aliases and thinking config for gemini-3.1 models

[anishs1207]

Summary

This PR resolves the INVALID_ARGUMENT (400) API error encountered when using Gemini 3.1 models. It ensures that all Gemini 3.1 variants are correctly mapped to the chat-base-3 configuration, which includes the mandatory thinkingLevel parameter required by the Google Generative AI API.

Details

The investigation revealed that while gemini-3.1-pro-preview was present, it lacked an entry in the aliases section of defaultModelConfigs.ts. This caused the CLI to fall back to a basic configuration that enabled "Thinking" mode but failed to provide a thinkingLevel (e.g., HIGH). Gemini 3.1 models strictly require this level when thinking is enabled; without it, the API rejects the request as an invalid argument.

Changes include:

• Added gemini-3.1-pro-preview, gemini-3.1-pro-preview-customtools, and gemini-3.1-flash-lite-preview to the core alias registry.
• Mapped these models to inherit from chat-base-3 to ensure ThinkingLevel.HIGH is included in all requests.

Related Issues

Fixes #26997, #27000, #27005, #26996

How to Validate

1. Run Tests: Execute the updated test suite in packages/core/src/config/models.test.ts to ensure 3.1 models resolve with the correct config.
   • npm run test core
2. Manual Verification: Use a Gemini 3.1 model to process a comparison prompt (e.g., "AMD Ryzen 9 9950X vs 9950X3D") and verify the 400 error no longer occurs.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • Windows (Validated via MINGW64/git-bash)
    • npm run

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses API errors encountered when using Gemini 3.1 models by ensuring they are correctly registered and mapped to the appropriate base configuration. By inheriting from chat-base-3, these models now correctly include the required thinkingLevel parameter, preventing invalid argument errors during API requests.

Highlights

• Model Configuration Updates: Added three new Gemini 3.1 models to the core alias registry: gemini-3.1-pro-preview, gemini-3.1-pro-preview-customtools, and gemini-3.1-flash-lite-preview.
• Configuration Mapping: Mapped the new Gemini 3.1 models to the chat-base-3 configuration to ensure the mandatory thinkingLevel parameter is correctly included, resolving API 400 errors.
• Test Coverage: Added comprehensive unit tests in models.test.ts to verify that all Gemini 3.1 models resolve to the expected configuration with the required thinkingLevel.

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[gemini-code-assist]

Code Review

This pull request adds configurations for new Gemini 3.1 models (gemini-3.1-pro-preview, gemini-3.1-pro-preview-customtools, and gemini-3.1-flash-lite-preview) to the default model configurations, extending the chat-base-3 base. It also includes unit tests to ensure these models correctly resolve their configurations, specifically verifying the thinkingLevel property. I have no feedback to provide.

[anishs1207]

Ready for review of PR @scidomino