feat: support SHOW DEFAULT_TRANSACTION_ISOLATION for PG databases

[olavloite]

The SHOW DEFAULT_TRANSACTION_ISOLATION command can be used in PostgreSQL to query the current default isolation level for a connection. This command was not supported by the Spanner JDBC driver and PGAdapter, which made it difficult for customers to check whether their attempt to set a different default isolation level actually worked.

Updates GoogleCloudPlatform/pgadapter#3984

[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 Spanner JDBC driver and PGAdapter by introducing support for the PostgreSQL SHOW DEFAULT_TRANSACTION_ISOLATION command. This feature allows users to easily inspect the configured default transaction isolation level, which was previously difficult to verify. Additionally, the behavior of SHOW TRANSACTION_ISOLATION_LEVEL has been refined to accurately reflect the isolation level of an active transaction, providing more precise information to users. These changes improve the diagnostic capabilities and overall compatibility with PostgreSQL-specific commands.

Highlights

• PostgreSQL SHOW DEFAULT_TRANSACTION_ISOLATION Support: The Spanner JDBC driver and PGAdapter now support the SHOW DEFAULT_TRANSACTION_ISOLATION command, allowing users to query the default isolation level for a connection.
• Improved SHOW TRANSACTION_ISOLATION_LEVEL Behavior: The SHOW TRANSACTION_ISOLATION_LEVEL command now correctly returns the current transaction's isolation level if a transaction is active, otherwise it returns the default isolation level.
• Client-Side Statement Definition: A new client-side statement definition has been added to PG_ClientSideStatements.json to recognize and handle SHOW DEFAULT_TRANSACTION_ISOLATION queries.

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[gemini-code-assist]

Code Review

This pull request adds support for the SHOW DEFAULT_TRANSACTION_ISOLATION command for PostgreSQL databases, which is a valuable feature for users to verify their connection's default isolation level. The implementation is clean and follows the existing pattern for client-side statements. Additionally, this PR includes a fix for SHOW TRANSACTION_ISOLATION_LEVEL to correctly report the current transaction's isolation level instead of a hardcoded value. The changes are well-structured and correct. I have one minor suggestion to improve code conciseness.