chore(spanner): implement location router and transaction affinity tracking

[olavloite]

Implements LocationRouter, the core connection resolution engine for location-aware routing in the Spanner Rust client.

[gemini-code-assist]

Code Review

This pull request introduces the LocationRouter module to handle Spanner client request routing, including transaction affinity tracking, key range cache lookups, endpoint cooldowns, and default fallback connections. The feedback suggests addressing a potential memory leak risk in the unbounded AffinityTracker by implementing a bounded cache or ensuring cleanup on drop, and simplifying a comparison expression for better readability.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the LocationRouter module in the Spanner client routing system, which resolves the optimal server connection for a given request context by evaluating transaction affinity, querying the key range cache, and bypassing endpoints currently on cooldown. A comprehensive suite of unit tests is also added to verify its behavior. There are no review comments, and I have no feedback to provide.

[codecov]

Codecov Report

❌ Patch coverage is 97.92285% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.25%. Comparing base (35b9590) to head (f20a36c).

Files with missing lines	Patch %	Lines
src/spanner/src/routing/location_router.rs	97.92%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6276    +/-   ##
========================================
  Coverage   96.24%   96.25%            
========================================
  Files         280      281     +1     
  Lines       72146    72483   +337     
========================================
+ Hits        69440    69770   +330     
- Misses       2706     2713     +7     

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