fix(config): don't rewrite explicit gemini-2.5-flash model selection

[TECHIE-TITAN]

Summary

• isFlashModel() used a broad model.endsWith('flash') match to decide which
  models get auto-upgraded to gemini-3.5-flash once 3.5 Flash GA is active for
  a backend. That also matched the explicitly pinned gemini-2.5-flash, so
  --model gemini-2.5-flash was silently rewritten to gemini-3.5-flash on
  backends (e.g. Vertex AI) where GA is assumed but not actually available,
  causing a ModelNotFoundError.
• Narrowed isFlashModel() to only match known flash aliases/pointers, not
  arbitrary pinned model IDs, and removed the equivalent auto-upgrade rule for
  gemini-2.5-flash in the experimental dynamic model-config table.

Fixes #29213

Test plan

• vitest run src/config/models.test.ts src/config/config.test.ts src/services/modelConfigService.test.ts — 371 passed
• npm run lint --workspace @google/gemini-cli-core — clean
• npm run typecheck --workspace @google/gemini-cli-core — clean
• Added regression test reproducing the exact issue Unexpected model resolution: gemini-2.5-flash is mapped to gemini-3.5-flash #29213 scenario

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[github-actions]

📊 PR Size: size/M

• Lines changed: 146
• Additions: +134
• Deletions: -12
• Files changed: 4

[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 an issue where the Gemini CLI was incorrectly and silently rewriting explicitly requested model IDs (specifically gemini-2.5-flash) to gemini-3.5-flash. This behavior caused failures for users whose environments did not have access to the newer model. The changes ensure that model resolution logic respects user-provided model identifiers while maintaining correct auto-upgrade behavior for generic flash aliases.

Highlights

• Model Resolution Logic: Updated the isFlashModel helper function to remove the broad endsWith('flash') check, preventing unintended automatic upgrades of explicitly pinned models like gemini-2.5-flash.
• Configuration Cleanup: Removed the legacy auto-upgrade rule for gemini-2.5-flash from the experimental dynamic model-config table.
• Regression Testing: Added comprehensive test cases to verify that explicitly requested model IDs are preserved and not silently rewritten during GA rollout scenarios.

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[gemini-code-assist]

Code Review

This pull request addresses issue #29213 where explicitly requested concrete flash models (such as gemini-2.5-flash) were being unexpectedly resolved/upgraded to gemini-3.5-flash. This was fixed by removing the gemini-2.5-flash mapping from defaultModelConfigs.ts and removing the generic endsWith('flash') check in isFlashModel within models.ts. Unit tests have been added to verify that explicitly requested model IDs remain untouched. I have no feedback to provide as there are no review comments.