Expand DB adapter test coverage and contracts

[nicosuave]

This adds thorough, per-adapter unit tests for DB adapters and new core tests.

• Per-adapter DB coverage: URL parsing edge cases, invalid URLs, executemany semantics, get_tables/get_columns SQL, query history SQL, result wrapper ordering/exhaustion, Arrow conversion (optional deps), and injection-surface checks.
• Added core tests for dialect parsing, time intelligence, schema generation, CLI wiring, and MCP errors.
• Added server tests for Postgres wire protocol helpers.

Tests:

• uv run --extra dev --extra lsp pytest -m 'not integration'