fix(spanner): error instead of panic for iterator after tx end

[olavloite]

Return an error instead of panic if a RowIterator is used after the transaction that created it is committed or rolled back.

[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 enhances the robustness of the Spanner client library by preventing a potential panic that could occur if a RowIterator was used after its underlying transaction had concluded. Instead of crashing, the system will now return a clear FailedPrecondition error, providing a more predictable and manageable experience for developers. This change improves error handling for a specific edge case related to transaction lifecycle management.

Highlights

• New Error Type: Introduced a new error function, errTransactionNoLongerActive, which returns a FailedPrecondition error when a transaction is no longer active.
• Panic Prevention: Modified the txReadOnly.query method to check if the transaction's session handle is nil (indicating an inactive transaction). If so, it now returns the errTransactionNoLongerActive error instead of potentially panicking.
• New Test Case: Added TestReadFromQueryAfterCommitOrRollback to verify that attempting to use a RowIterator after its associated transaction has been committed or rolled back correctly results in the new FailedPrecondition error.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a panic that could occur when a RowIterator is used after its associated transaction has been closed. The fix involves adding a check for an active transaction before using it and returning a FailedPrecondition error instead. The accompanying test case is thorough, covering both commit and rollback scenarios. I've added one suggestion to improve the test structure for better readability and diagnostics.