chore(spanner): implement cache updater for location-aware routing

[olavloite]

Implement CacheUpdater to coordinate between Spanner CacheUpdate wire messages and the client's in-memory routing table (KeyRangeCache) and connection pool (ConnectionCache).

[gemini-code-assist]

Code Review

This pull request introduces the CacheUpdater module to handle location-aware routing cache updates by applying CacheUpdate payloads to the KeyRangeCache and pre-warming server connections in the ConnectionCache. The review feedback recommends using a safer, error-returning method like try_get instead of get to avoid potential panics when pre-warming connections, as well as logging errors instead of swallowing them. Additionally, it suggests replacing flaky tokio::task::yield_now() calls in unit tests with a robust polling mechanism with a timeout.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the CacheUpdater module to the Spanner location-aware routing implementation. CacheUpdater coordinates the ingestion of CacheUpdate payloads, applying updates to the in-memory KeyRangeCache and asynchronously pre-warming server connections in the ConnectionCache using the Tokio runtime. A comprehensive suite of unit tests has been added to verify its behavior, including deduplication and generation updates. There are no review comments to address, and the implementation aligns with the repository's style guidelines.

[codecov]

Codecov Report

❌ Patch coverage is 99.15014% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.26%. Comparing base (35b9590) to head (3a3013d).

Files with missing lines	Patch %	Lines
src/spanner/src/routing/cache_updater.rs	99.15%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6271      +/-   ##
==========================================
+ Coverage   96.24%   96.26%   +0.01%     
==========================================
  Files         280      281       +1     
  Lines       72146    72499     +353     
==========================================
+ Hits        69440    69790     +350     
- Misses       2706     2709       +3     

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