feat: support PG connection properties in driver

[surbhigarg92]

Summary

Adds support for PostgreSQL-specific GUC configuration properties (application_name, timezone, client_min_messages, client_encoding, default_transaction_isolation, server_version, and server_version_num). These defaults and no-op properties enable standard PostgreSQL drivers to connect and execute initial setup queries seamlessly.

Changes

• PostgreSQL GUC Properties: Defined read-only and no-op PostgreSQL compatibility properties.
• Dialect Isolation: Segregated PostgreSQL properties from GoogleSQL connections so GoogleSQL connection states remain clean.
• Performance Optimization: Pre-computed postgresConnectionProperties during package init() to avoid map allocations and copying on every connection setup.
• Test Coverage: Added mock server unit tests in conn_with_mockserver_test.go verifying read/write behavior, read-only validation, and dialect restriction.

[gemini-code-assist]

Code Review

This pull request introduces PostgreSQL compatibility connection properties (GUCs) and corresponding mock server tests for the Spanner PostgreSQL dialect. The feedback suggests optimizing performance by pre-computing the merged PostgreSQL connection properties map during package initialization, which avoids map allocation and copying overhead on every connection creation.

[surbhigarg92]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces PostgreSQL compatibility connection properties (GUCs) such as application_name, client_min_messages, client_encoding, timezone, default_transaction_isolation, server_version_num, and server_version for the PostgreSQL dialect, along with corresponding tests. Feedback recommends using Go subtests (t.Run) to isolate distinct test cases in accordance with the repository style guide, and adding missing verifyShowFails assertions to align with the test's documented behavior.

[surbhigarg92]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds PostgreSQL compatibility connection properties (GUCs) such as application_name, client_min_messages, client_encoding, timezone, default_transaction_isolation, server_version_num, and server_version to support client library setup queries when using the PostgreSQL dialect. The review feedback suggests making default_transaction_isolation writable (validating that it is set to serializable) instead of strictly read-only, as standard PostgreSQL drivers or ORMs may attempt to set this parameter during initialization. Additionally, the feedback recommends adding a test case to verify that setting it to 'serializable' succeeds and updating the corresponding subtest name.

[surbhigarg92]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces PostgreSQL compatibility connection properties (GUCs) such as application_name, client_min_messages, client_encoding, timezone, default_transaction_isolation, server_version_num, and server_version to support client library setup queries when using the PostgreSQL dialect. It also adds corresponding unit tests. The review feedback correctly identifies that the newly acquired database connections in the tests are not closed, which could lead to connection leaks, and suggests using defer silentClose(conn) to resolve this.