chore(spanner): record transaction affinity for streaming RPCs with inline begin

[olavloite]

When executing a streaming query (execute_streaming_sql) or streaming read (streaming_read) with an inline-begin read-write transaction in Spanner Omni, the returned transaction ID was not being captured to establish location-aware routing affinity.

This change:

• Captures the transaction ID from the first stream response chunk and registers node affinity with the location router.
• Introduces a single-use TransactionIdCallback (Box) attached during stream construction.
• Adds helper logic on DatabaseClient to determine whether affinity recording is required based on the transaction selector and routing mode.
• Expands mock tests covering inline-begin streaming queries, streaming reads, stream errors, and aborted transaction retries.

[gemini-code-assist]

Code Review

This pull request implements transaction affinity routing for Spanner server-streaming RPCs (ExecuteStreamingSql and StreamingRead) by introducing a callback mechanism to capture the first transaction ID returned in a stream. It also adds comprehensive integration and unit tests to verify affinity recording, error handling, aborted retries, and tablet routing. The review feedback suggests refactoring several conditional builder configurations to use idiomatic Rust expressions instead of mutable variables.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.81%. Comparing base (bd3124b) to head (ffc8d98).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6731    +/-   ##
========================================
  Coverage   96.80%   96.81%            
========================================
  Files         313      313            
  Lines      104285   104513   +228     
========================================
+ Hits       100958   101188   +230     
+ Misses       3327     3325     -2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements transaction affinity recording for streaming RPCs (such as streaming SQL and streaming reads) that initiate inline read-write transactions. It introduces a TransactionIdCallback mechanism to extract the transaction ID from the first PartialResultSet in the stream and record affinity to the target server address. Additionally, comprehensive unit and mock integration tests have been added to verify this behavior across various scenarios, including multi-chunk streams, aborted transaction retries, and tablet routing. I have no feedback to provide as there are no review comments to evaluate.