feat(dbapi): wire timeout parameter through Connection to execute_sql

[sakthivelmanii]

• Wire timeout parameter to Connection.init, connect(), run_statement(), and validate().
• Pass timeout from connection to execute_sql and batch_update calls when specified.
• Add unit tests for DBAPI timeout propagation.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces timeout support for Spanner DB-API operations, allowing users to specify a timeout duration for connections and individual SQL statements. It updates the Connection, Cursor, and batch_dml_executor modules to propagate this timeout value and includes comprehensive unit tests. The reviewer suggests refactoring the _do_batch_update_autocommit function to accept the timeout parameter directly instead of the cursor object to improve decoupling and simplify parameter handling.