Implement GridFS upload streams and helpers

[schapman1974]

Support chunked file uploads through PyMongo-shaped APIs.

Initial API:

• open_upload_stream
• open_upload_stream_with_id
• upload_from_stream
• upload_from_stream_with_id

Acceptance criteria:

• Accept bytes, file-like streams, and incremental writes.
• Split content into numbered chunk documents using the configured chunk size.
• Write the file metadata only after a successful upload.
• Abort cleanly and remove partial chunks after errors or explicit abort.
• Enforce filename, metadata, identifier, BSON, and chunk-size validation.
• Return compatible file identifiers and stream properties.
• Add empty, exact-boundary, multi-chunk, and interrupted-upload tests.