test: expand gaussdb integration coverage

[lewiszlw]

Summary

• expand the GaussDB integration suite to match the core postgres/mysql coverage patterns
• add GaussDB cases for timestamp decoding, metadata queries, pushdown behavior, projections, empty tables, inserts, auto-increment inserts, and pool cleanup
• extend the GaussDB init fixture with the extra tables and sequence grants those tests need

Testing

• cargo fmt --all --check
• cargo test -p integration-tests --no-run
• cargo clippy --workspace --all-targets --all-features -- -D warnings