Stop collapsing three-character drive-relative paths to the drive root

[itzzdev09]

Problem

On Windows, make_path_posix turns a three-character drive-relative path into the drive root, dropping the name:

>>> from fsspec.implementations.local import make_path_posix
>>> make_path_posix("C:x")
'C:/'            # a different, real location; "x" is gone
>>> make_path_posix("C:xy")
'C:xy'           # a longer drive-relative path keeps its name
>>> make_path_posix("C:.")
'C:/'

C:x names x relative to the current directory on drive C (ntpath.abspath("C:x") resolves it that way). Returning C:/ is worse than leaving the path alone: it looks absolute and valid, so a call like fs.rm("C:x") or fs.ls("D:a") quietly operates on the drive root instead of failing or using the intended path.

Cause

if path[1:2] == ":":
    if len(path) <= 3:
        # nt root (something like c:/)
        return path[0] + ":/"

The length check is meant for the roots c:, c:/ and c:\, but any three-character path with a colon in second place matches, including c:x, D:a and e:..

Change

Only treat X: and X: followed by / or \ as the root. A three-character drive-relative path now falls through to the same handling as a longer one, so C:x behaves like C:xy.

This deliberately does not make drive-relative paths absolute. test_make_path_posix_returns_absolute_paths already marks "f:foo" as xfail(WIN, reason="unsupported"), and this change leaves that case as it was. It only removes the silent redirect to the root.

Tests

test_make_path_posix_short_drive_relative_path_is_not_the_root, Windows-only, for c:x, D:a and e:.. All three fail against master and pass with the change.

test_local.py: 160 passed, 4 skipped, 8 xfailed — the existing root cases (C:\, e:) still resolve to the root, and the f:foo xfail is unchanged, with no unexpected passes.

ruff 0.14.3 (the pre-commit pin), ruff format and codespell are clean.

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