IO: Fix Windows drive letters misidentified as URI schemes

[qzyu999]

Rationale

On Windows, urlparse('C:\Users\...') produces scheme='c', causing:

ValueError: Unrecognized filesystem type in URI: c

This breaks all local file operations for Windows users.

The Fix

A single platform-guarded predicate shared across all three affected parse sites:

def _is_windows_drive_letter(scheme: str) -> bool:
    return sys.platform == 'win32' and len(scheme) == 1 and scheme.isalpha()

When detected, the scheme is remapped to 'file', which routes to PyArrow/fsspec's local filesystem which already handles Windows paths natively.

Changes

File	Change
pyiceberg/io/__init__.py	Add predicate + normalize scheme in _infer_file_io_from_scheme
pyiceberg/io/pyarrow.py	Guard in parse_location
pyiceberg/io/fsspec.py	Guard in _get_fs_from_uri
tests/io/test_io.py	Predicate + integration tests
tests/io/test_pyarrow.py	parse_location Windows test

Why all three sites?

They execute at different lifecycle stages and are reachable through independent code paths. Fixing only one (as PR #3161 did) leaves the others broken.

Regression safety

The sys.platform == 'win32' guard means this is literally invisible on Linux/macOS the predicate short-circuits to False without inspecting the scheme. Existing CI on ubuntu-latest cannot observe any change.

Are these changes tested?

Yes. 11 new test cases covering:

• Predicate correctness (real schemes False, drive letters on Windows True)
• Integration (Windows paths resolve to PyArrowFileIO)
• Platform-conditional via @pytest.mark.skipif

All existing tests continue to pass.

Closes

Closes #3720
Related: #2477, #1005

[rambleraptor]

Windows newb here:

I'm looking at https://stackoverflow.com/questions/446209/possible-values-from-sys-platform. There's a couple other Windows derivatives mentioned. Should we do that for these too?

[qzyu999]

Good question! My understanding is that "win32" is sufficient here, the drive letter parsing bug only exists on native Windows where paths look like C:\....

The other sys.platform values from that SO thread don't need coverage because they use POSIX paths, so urlparse never produces a single-letter scheme on them:

• Cygwin (sys.platform == "cygwin") translates C:\Users\... to /cygdrive/c/Users/... at the filesystem layer (Cygwin docs)
• MSYS2 (sys.platform == "msys") similarly maps C:\Users to /c/Users (MSYS2 docs)

sys.platform == "win32" is also the standard CPython idiom for this, it's how the stdlib distinguishes Windows from POSIX behavior (sys.platform docs), and what pytest recommends for skipif markers.