feat(spanner): support read-only BeginTransaction request options

[olavloite]

Adds support for setting GAX request options for the BeginTransaction RPC call for read-only transactions. These options are only used if the transaction uses the ExplicitBegin transaction option.

Updates #4972

[gemini-code-assist]

Code Review

This pull request introduces configuration options (attempt timeout, retry policy, and backoff policy) for the BeginTransaction RPC in read-only transactions. The review feedback highlights a critical regression where initializing these options to a default value by default causes the fallback mechanism to discard custom query-level options. The reviewer suggests changing the options field to an Option type initialized to None, lazily initializing it only when explicitly configured, and updating the corresponding tests.

[codecov]

Codecov Report

❌ Patch coverage is 99.70501% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.89%. Comparing base (73cd277) to head (3b7e5d1).

Files with missing lines	Patch %	Lines
src/spanner/src/read_only_transaction.rs	99.70%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5785      +/-   ##
==========================================
+ Coverage   97.88%   97.89%   +0.01%     
==========================================
  Files         226      226              
  Lines       56465    56801     +336     
==========================================
+ Hits        55273    55608     +335     
- Misses       1192     1193       +1     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces configuration options for the BeginTransaction RPC in read-only transactions, allowing users to customize attempt timeouts, retry policies, and backoff policies. A review comment points out that when custom transaction-level options are configured, they completely overwrite statement-specific fallback options (such as custom timeouts) instead of merging with them. The reviewer suggests merging the options to avoid discarding statement-specific configurations.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the ability to configure custom request options (such as attempt timeouts, retry policies, and backoff policies) specifically for the BeginTransaction RPC in read-only transactions. It adds builder methods to MultiUseReadOnlyTransactionBuilder to set these options, merges them with statement-level options during fallback explicit begins, and includes comprehensive unit tests to verify the behavior. There are no review comments to address, and the implementation is clean, well-tested, and adheres to the repository's style guidelines.