test(spanner): add mock server tests for location-aware routing

[olavloite]

Adds an extensive test set using a mock Spanner server to verify that location-aware routing is behaving as expected. This test suite is not only intended to verify the current behavior, but also to guard against accidental regressions in the future.

[gemini-code-assist]

Code Review

This pull request adds a comprehensive suite of mock server integration tests (mock_tests.rs) to verify Spanner's location-aware routing and multi-server topologies, and registers the new module in mod.rs. The review feedback consistently points out violations of the repository style guide regarding error handling in tests, specifically recommending the use of the ? operator instead of .expect() across various helper functions and test cases to propagate errors cleanly.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive suite of mock server integration tests in src/spanner/src/routing/mock_tests.rs to verify Spanner's location-aware routing and multi-server topologies. The tests cover inline cache updates, direct multi-server routing, replica selection, cooldown/failover behaviors, dynamic range updates, transaction affinity isolation, and proactive background cache synchronization. Additionally, the new test module is registered in src/spanner/src/routing/mod.rs. I have no further feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.36%. Comparing base (9ed9339) to head (1669eed).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6465      +/-   ##
==========================================
- Coverage   96.37%   96.36%   -0.01%     
==========================================
  Files         297      297              
  Lines       84017    84017              
==========================================
- Hits        80971    80967       -4     
- Misses       3046     3050       +4     

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