Add comprehensive tests for SigMFCollection

[Copilot]

SigMFCollection had almost no test coverage despite being a core part of the API. This PR adds 28 new tests across four classes covering round-trip serialization, schema validation, and common use cases.

New test classes

• TestCollectionConstructor — empty constructor and metadata-initialized constructor
• TestCollectionRoundTrip — write/read round-trip verifying metadata fields, get_collection_info(), and raw JSON structure (stream name/hash entries)
• TestCollectionValidation — schema-valid cases (minimal, all optional fields, extensions array) and invalid cases (missing core:version, missing collection top-level key)
• TestCollectionCommonUseCases — covers:
  • __len__, get_stream_names
  • get_SigMFFile by index, by name, and with an unknown name (returns None)
  • set_collection_field / get_collection_field including default value fallback
  • set_collection_info / get_collection_info
  • set_streams hash recomputation
  • skip_checksums=True on the constructor
  • Overwrite protection (SigMFFileExistsError) and overwrite=True
  • verify_stream_hashes — passes on valid hashes, raises SigMFFileError on corrupted hash
  • Error cases: non-existent metafile, wrong file extension
  • dumps() produces valid JSON with expected structure