fix(core): preserve explicit flash model IDs

[sylvesterkaczmarek]

Fixes #28859

Summary

• restrict the Gemini 3.5 Flash rollout rewrite to the generic flash alias and known rollout IDs
• preserve explicit model IDs such as gemini-3.6-flash and gemini-3.7-flash
• allow invalid explicit IDs to pass through so the API rejects them instead of silently substituting another model
• add focused regression coverage

Why

The static resolver could rewrite explicit model IDs ending in flash to the Gemini 3.5 Flash rollout target. This made published models unreachable by name and allowed nonexistent model IDs to appear to work.

Validation

Added focused unit coverage in packages/core/src/config/models.explicit-flash.test.ts.

Opening as draft because the full repository preflight was not available in this environment.

[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 refines the static model resolver to ensure that explicit model IDs are not incorrectly rewritten to the Gemini 3.5 Flash target. By narrowing the scope of the rollout logic, the system now correctly preserves specific model versions while still allowing the generic 'flash' alias to be promoted as intended.

Highlights

• Model Resolution Logic: Updated the model resolution logic to restrict the Gemini 3.5 Flash rollout to specific aliases and known rollout IDs, preventing the unintended rewriting of explicit model IDs.
• Removal of Deprecated Helper: Removed the isFlashModel helper function as it was no longer needed with the more precise resolution criteria.
• Regression Testing: Added a new test suite in packages/core/src/config/models.explicit-flash.test.ts to ensure explicit model IDs are preserved correctly during the rollout.

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[github-actions]

📊 PR Size: size/M

• Lines changed: 68
• Additions: +55
• Deletions: -13
• Files changed: 2

[gemini-code-assist]

Code Review

This pull request refactors the model resolution logic in packages/core/src/config/models.ts by removing the isFlashModel helper function and explicitly checking for specific flash model identifiers and aliases during the Gemini 3.5 Flash rollout. It also introduces a new test suite in models.explicit-flash.test.ts to verify that explicit flash model IDs are preserved correctly. No review comments were provided, so there is no additional feedback.