Add cross-backend and wire-client GridFS contracts

[schapman1974]

Prove that GridFS behavior remains compatible across TinyMongo backends and future MongoDB clients.

Acceptance criteria:

• Run shared GridFS contracts against memory, JSON, SQLite, DuckDB, and Parquet where applicable.
• Compare file metadata, chunks, errors, and stream behavior with real MongoDB.
• Test PyMongo GridFS against TinyMongo wire-server mode once read and write commands are available.
• Verify Compass can display the underlying files and chunks collections without corrupting them.
• Include files larger than the BSON document limit without making normal CI excessively large.
• Publish backend limitations and the tested GridFS compatibility matrix.