fix: session leak for empty transaction with tag

[olavloite]

Executing an empty read/write transaction with a tag would cause a session leak, as the transaction was marked as not owned.

Fixes #635

[gemini-code-assist]

Summary of Changes

Hello @olavloite, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical session leak that could occur when an empty read/write transaction with a tag was executed, due to incorrect transaction ownership tracking. The changes ensure that all initiated transactions are properly owned, preventing resource exhaustion. Additionally, it introduces safeguards to prevent the creation of multiple concurrent transactions on a single connection, enhancing the stability and predictability of transaction management within the Spanner Entity Framework Core provider.

Highlights

• Session Leak Fix: Resolved a session leak issue that occurred when executing empty read/write transactions with tags, by ensuring transactions are correctly marked as owned.
• Transaction Management Robustness: Implemented a check to prevent multiple transactions from being initiated on the same connection, improving transaction management robustness.
• Consistent Transaction Ownership: Enhanced transaction initiation methods across various BeginTransaction overloads to consistently manage transaction ownership and state.
• New Unit Tests: Added new unit tests to validate the fix for session leaks in empty tagged transactions and to confirm proper error handling when attempting to start multiple transactions.

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[gemini-code-assist]

Code Review

This pull request addresses a critical session leak issue by ensuring proper transaction management within the SpannerRelationalConnection. The changes introduce explicit checks to prevent multiple active transactions on a single connection and centralize transaction creation logic. New tests have been added to validate that empty read/write transactions with tags and attempts to start multiple transactions on the same connection do not lead to session leaks. Additionally, CancellationToken is now correctly propagated in asynchronous transaction methods, improving API consistency.