Implement _get_file_stats utility for consistent file metadata retrieval

[varma1221]

What problem does this solve?
Currently, retrieving basic file metadata (like size or protocol) requires repeating _init_filesystem and fsspec boilerplate across different modules. This leads to inconsistent handling of return types especially for the protocol attribute, which some backends return as a string and others as a list and lacks centralized validation for mandatory fields like file size.

How does it solve it?
This PR introduces a private utility _get_file_stats in util.py. It:

• Standardizes the retrieval of size, mtime, and protocol into a single dictionary.
• Normalizes fs.protocol into a single string regardless of the backend implementation.
• Implements strict validation on file size, raising a ValueError instead of returning None, which prevents hard-to-debug crashes in downstream mathematical
  operations.
• Simplifies I/O logic for future modules by providing a clean, protocol-agnostic metadata interface.

Relevant issue numbers
Part of ongoing improvements to internal I/O utilities.

Testing done

• Added comprehensive unit tests in tests/test_util.py.
• Verified local file metadata retrieval (size, mtime, path).
• Verified mock behavior for remote protocols (GCS/S3) and protocol normalization.
• Verified that missing size metadata correctly triggers a ValueError.
• Confirmed that standard FileNotFoundError propagates correctly.
• Ran mypy and pre-commit to ensure type safety and code quality.

Breaking changes
None. This is an internal utility addition.