remove the legacy feature flag of gemini 3

[shixiao-coder]

The gemini 3 feature flag was use when we roll up from 2.5 to 3, it's no longer needed, all non prod env has removed this feature flag

[gemini-code-assist]

Summary of Changes

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This pull request removes the legacy 'enable_gemini_3_flash' feature flag from the production configuration. Since the transition to Gemini 3 is complete and the flag has already been removed from non-production environments, this change cleans up the codebase by eliminating redundant configuration.

Highlights

• Cleanup of legacy configuration: Removed the 'enable_gemini_3_flash' feature flag from the production configuration file as it is no longer required.

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[github-actions]

⚠️ PRODUCTION FEATURE FLAG CHANGE — REVIEW CHECKLIST

Please conduct a careful review before merging this production change & coordinate with the oncaller.
Merging this PR will trigger a rolling restart of all Kubernetes pods in production.

Things to check before approving:

• Staging Validated: Have these exact flag changes been tested and verified in staging?
• Cache Invalidation: If API responses change, remember to clear the Redis cache after merging.
• Time Window: Ensure updates occur during global working hours; request lead approval for Friday–Sunday updates.
• Oncall Communication: Please additionally announce this update on the oncall thread.
• Type Safety: Verify that flag value types match what the server binary expects.

[gemini-code-assist]

Code Review

This pull request removes the deprecated "enable_gemini_3_flash" feature flag from the production configuration file. There are no review comments, and I have no feedback to provide.