feat(sqlalchemy-spanner): wire timeout execution option through to DBAPI Connection.timeout

[sakthivelmanii]

• Reset connection timeout to None on pool checkin in reset_connection.
• Support timeout execution option in SpannerExecutionContext.pre_exec.
• Restore baseline connection timeout in post_exec and handle_dbapi_exception to prevent statement-level timeout leakage across queries on the same connection.
• Add unit tests for execution options timeout lifecycle.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces support for statement-level timeouts in the SpannerExecutionContext by saving the original connection timeout before execution, applying the execution option's timeout, and restoring the original timeout in post_exec and handle_dbapi_exception. It also resets the timeout to None when resetting the connection. The review feedback suggests wrapping the timeout restoration logic in a try...except block to prevent secondary exceptions (e.g., on a broken connection) from masking the primary database exception.