test(spanner): add integration tests for Batch DML

[olavloite]

• Adds integration tests for executing Batch DML on Spanner.
• Fixes an issue in the BeginTransaction handling for Batch DML on the Emulator, as the Emulator can return Aborted even for BeginTransaction (this happens when there already is an active transaction on the Emulator)
• Fixes an issue where the transaction ID would not be extracted from the ExecuteBatchDmlResponse if the response returned a non-zero status code. This indicates that an error occurred for one of the statements in the batch (but not for the first statement).

[gemini-code-assist]

Code Review

This pull request implements robust error handling for Batch DML operations in Spanner, specifically addressing cases where service-level errors are returned within successful gRPC responses. It introduces a CheckServiceError trait to normalize these responses and updates the execute_with_retry! macro to handle transaction state resets correctly during failures. Additionally, the PR includes comprehensive integration tests for various Batch DML scenarios and refactors response processing for better readability. Feedback was provided regarding the use of unwrap() in production code, which violates the project's style guide.

[codecov]

Codecov Report

❌ Patch coverage is 99.04459% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.89%. Comparing base (b6b59db) to head (16ba8ef).

Files with missing lines	Patch %	Lines
src/spanner/src/read_write_transaction.rs	98.90%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5723      +/-   ##
==========================================
+ Coverage   97.88%   97.89%   +0.01%     
==========================================
  Files         226      226              
  Lines       55691    55985     +294     
==========================================
+ Hits        54512    54806     +294     
  Misses       1179     1179              

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request enhances the Spanner client's handling of Batch DML and lazy transaction starts. It introduces a LazyTransactionStartGuard to manage state transitions during inline begins and a CheckServiceError trait to identify service-level errors within successful RPC responses. The execute_with_retry! macro was refactored to utilize these components, ensuring more reliable transaction recovery and retries, particularly for aborted errors. Comprehensive integration tests for various Batch DML failure and continuation scenarios were also added. Reviewer feedback suggests further simplifying the macro logic by adopting more idiomatic Rust patterns, such as using Option combinators and the ? operator to handle result propagation and error extraction more concisely.