feat(c/driver/postgresql): add fixture-backed NUMERIC round-trip tests

[Mandukhai-Alimaa]

• Add adbc.postgresql.disable_decimal_fast_path, an undocumented option to disable the fast implementation as a test knob.
• Add a script that generates a CSV fixture exhaustively enumerating every coefficient across all valid scales for a small precision, checked into the repo.
• Add a round-trip test that ingests the fixture and asserts PostgreSQL echoes back the exact expected text, run once per path (fast/slow).

Closes #4528