chore(spanner): implement transaction affinity resolution, R/W counting, and CAS conflict handling in channel pool

[olavloite]

Implements transaction affinity resolution and concurrency handling for multi-statement transactions inside ChannelPool.

Key changes:

• Tracks active Read/Write transactions on channels using RAII guards so channels undergoing scale-down draining are kept alive until in-flight transactions complete.
• Implements affinity resolution supporting hard stickiness (Read/Write transactions follow their pinned channel even when draining) and soft stickiness (Read-Only transactions prefer warmth but cleanly fail over to active channels).
• Handles concurrent CAS races when multiple statements attempt to pin the same transaction at once, ensuring all statements consistently route to the winning channel.

[gemini-code-assist]

Code Review

This pull request refactors the Spanner channel pool and transaction affinity management. Key changes include tracking RwTransactionAffinityGuard within TransactionAffinity to manage active read/write transaction counts, introducing a robust compare-and-swap (CAS) conflict resolution mechanism (resolve_cas_conflict) for concurrent transaction pinning, simplifying ChannelEntry by removing redundant fields, and implementing Deref for ChannelLease. The feedback identifies an unresolved rustdoc link in the documentation of default_channel referencing ChannelPool::next_channel instead of pick_channel, which should be corrected to prevent CI build failures.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.03%. Comparing base (0b4b459) to head (dfca786).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6737      +/-   ##
==========================================
+ Coverage   97.00%   97.03%   +0.03%     
==========================================
  Files         326      326              
  Lines      107038   107430     +392     
==========================================
+ Hits       103827   104241     +414     
+ Misses       3211     3189      -22     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors and robustifies the Spanner channel pool's transaction affinity and channel leasing mechanisms. Key changes include introducing RwTransactionAffinityGuard management within TransactionAffinity to prevent premature channel reclamation during active read-write transactions, implementing transparent Deref to Channel for ChannelLease, and refactoring the compare-and-swap (CAS) conflict resolution logic in ChannelPool::resolve_affinity into a dedicated, loop-based resolve_cas_conflict method. Additionally, ChannelEntry was simplified by removing the redundant logical_channel_id field in favor of a getter, and extensive unit tests were added to verify the new guard behaviors and CAS resolution paths. I have no further feedback to provide as the changes are well-structured, idiomatic, and thoroughly tested.