chore(spanner): wire location-aware routing into begin_transaction and commit

[olavloite]

Connect unary BeginTransaction and Commit RPCs to location-aware routing in DatabaseClient:

• pre_route_begin_transaction / post_route_begin_transaction: Resolve route and attach covering RoutingHint when mutation_key is present. Bind transaction affinity to the resolved connection (or fallback default gateway for unkeyed begins) upon successful transaction creation.
• pre_route_commit / post_route_commit: Prioritize registered transaction affinity while deriving mutation RoutingHint from request.mutations. Fall back to round-robin channel pooling when neither affinity nor a routing key exists. Clear affinity upon commit completion.
• Mutation::select_mutation_key_ref: Add an O(1) fast path for single-mutation slices, operate on borrowed references, exclude unroutable Delete mutations (full-table deletes, empty key sets, or missing key sets), and extract is_routable_non_insert to keep selection logic flat and idiomatic.

[gemini-code-assist]

Code Review

This pull request refactors location-aware routing in the Spanner client to intercept unary BeginTransactionRequest and CommitRequest calls, resolving leader or affinity routing and attaching a RoutingHint when active. It introduces Mutation::select_mutation_key_ref to select routing keys by reference, avoiding unnecessary clones, and adds comprehensive unit tests covering various routing, affinity, and mutation selection scenarios. No review comments were provided, so there is no feedback to address.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.75%. Comparing base (f02b42c) to head (caa859c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6671      +/-   ##
==========================================
+ Coverage   96.72%   96.75%   +0.03%     
==========================================
  Files         313      313              
  Lines      101266   102368    +1102     
==========================================
+ Hits        97948    99049    +1101     
- Misses       3318     3319       +1     

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