add the use_config_threshold_for_spanner_embedding feature flag to prod following the flag for other env setup

[shixiao-coder]

This PR add the feature flag in prod environment and default to false

When false, we will read default value 0.7 from the code,
When true, we will set config value to be 0.6 based on test result https://docs.google.com/spreadsheets/d/12d9BPJagWS5z2WA8En9JY71TyGrCYpIBQ4LqI0JJ2v4/edit?gid=1132426848#gid=1132426848

[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 introduces a new feature flag to the production environment to control the threshold logic for Spanner embeddings. By gating this functionality, the team can safely toggle between the hardcoded default threshold and a configurable value based on recent testing results.

Highlights

• Feature Flag Addition: Added the 'use_config_threshold_for_spanner_embedding' feature flag to the production configuration.
• Default Configuration: Set the default state of the new feature flag to 'false' to ensure backward compatibility with the existing threshold logic.

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[gemini-code-assist]

Code Review

This pull request adds a new feature flag configuration 'use_config_threshold_for_spanner_embedding' to the production environment, which is initially disabled. This flag gates the use of the 'SPANNER_EMBEDDING_THRESHOLD' configuration. There are no review comments, and I have no feedback to provide.

[gmechali]

Can you ping us when your PR made it to production github.com//pull/6533
So we can approve this then. A bit too early for this PR :)

[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.

[gmechali]

approved, thanks!