chore(spanner): wire location-aware routing into streaming read and query RPCs

[olavloite]

• Add channel_id directly to Channel to avoid redundant modulo arithmetic
  when attaching Spanner request ID headers.
• Implement protobuf-level key extraction helpers in key_extractor
  (extract_proto_read_request_routing_key, extract_key_from_proto_key_set)
  supporting point keys and key ranges (start_closed/start_open).
• Add resolve_routing_connection to DatabaseClient and wire it into
  streaming_read and execute_query RPC dispatch.
• Keep location-aware routing disabled by default, gating activation behind the
  GOOGLE_SPANNER_EXPERIMENTAL_LOCATION_API environment variable.

[gemini-code-assist]

Code Review

This pull request refactors the Spanner client to pass direct Channel references instead of channel_hint indices, simplifying request ID generation by storing a 1-based channel_id on the Channel struct. It also introduces location-aware routing for streaming reads by extracting routing keys from ReadRequest to resolve optimal server connections. Feedback on these changes includes resolving a missing import for ProtoKeyRange in tests, optimizing resolve_routing_connection to return early when no routing information is present, and removing the now-unused next_channel_hint function.

[codecov]

Codecov Report

❌ Patch coverage is 99.28910% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.41%. Comparing base (1da1b78) to head (204cf59).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spanner/src/routing/key_extractor.rs	97.59%	4 Missing ⚠️
src/spanner/src/client.rs	96.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6494      +/-   ##
==========================================
+ Coverage   96.39%   96.41%   +0.02%     
==========================================
  Files         300      300              
  Lines       84585    85343     +758     
==========================================
+ Hits        81533    82283     +750     
- Misses       3052     3060       +8     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors the Spanner client to support location-aware routing by passing Channel references directly to RPC methods instead of numeric channel hints. It introduces connection resolution logic based on transaction IDs and routing keys, and adds comprehensive tests to verify both standard round-robin channel pooling and location-aware routing. Feedback on these changes highlights two key issues: first, use_transaction_affinity is hardcoded to false in RoutingContext, which disables transaction affinity routing even when a transaction ID is present; second, errors during key extraction are silently swallowed with .ok(), violating the repository style guide's rule against swallowing errors.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors the Spanner client's channel selection and request ID attachment by passing direct Channel references instead of numeric hints, storing a 1-based channel_id directly on the Channel struct. It also introduces location-aware routing for Spanner Omni, enabling the DatabaseClient to resolve optimal server connections using transaction affinity and routing keys extracted from read requests, while maintaining standard round-robin channel pooling as a fallback. I have reviewed the changes and have no feedback to provide as there are no review comments and the implementation is robust and well-tested.