chore(spanner): integrate location aware routing with RPCs

[rahul2393]

• Add locationAwareSpannerClient wrapper that intercepts RPCs and routes
  them to the server endpoint resolved by the existing channelFinder,
  falling back to the default gRPC channel when no endpoint is available
• Add endpointClientCache that creates and caches per-address gRPC
  connections
• Add transaction affinity tracking so Commit/Rollback route to the same
  server that handled the transaction, with read-only transactions routed
  independently per-request based on key ranges
• Move request preparation (prepareReadRequest, etc.) and response
  observation (observePartialResultSet, etc.) from transaction/batch
  code into the client wrapper, keeping routing concerns in one place

[gemini-code-assist]

Code Review

This pull request introduces a significant and well-structured feature for location-aware routing in the Spanner client. It adds a locationAwareSpannerClient wrapper to intercept RPCs, an endpointClientCache for managing per-endpoint connections, and logic for transaction affinity. The changes are extensive but appear to be well-tested and follow good design patterns by centralizing routing concerns. My main feedback is a potential issue regarding context propagation when creating new endpoint connections, which could lead to hangs if an endpoint is unresponsive. The original comment regarding context.Background() has been retained as it does not conflict with any provided rules.