test(refs): keep PathLike reference names portable on Windows

`test_revision_arguments_preserve_pathlike_paths` fails in the Windows
`Python package` jobs because `Path("refs/heads/main")` emits
`refs\heads\main`. Git rejects backslashes in reference names, so the
test raises `ValueError: Invalid reference` before exercising revision
argument handling. All four combinations of SHA-1/SHA-256 and
files/reftable fail. The same failures are present in the final Windows
Python 3.14t job for the introducing PR #2336, so this is inherited by
PR #2339 rather than caused by its blame-content change.

Use `PurePosixPath` for the reference-name test parameter. Reference
names use forward slashes on every platform; filesystem paths elsewhere
in the tests continue to use `Path`. Keep `PathLikeMock` coverage and
the existing reference validation unchanged.

Validation: 28 focused reference/revision tests pass on each of the
`cli` and `gix` backends, including all eight PathLike cases, nested
merge-base operands, reflog walks, fork-point lookup, and remote archives.
Both blame-content regressions also pass on each backend. Ruff 0.17.2
lint and formatting checks pass for `test/test_refs.py`; `git diff --check`
passes. Windows validation is left to the PR's CI matrix.

Agent-Model: unknown

Author: codex

test/test_refs.py

@@ -10,7 +10,7 @@
 import tarfile
 import unittest
 from itertools import chain
-from pathlib import Path
+from pathlib import Path, PurePosixPath
 from unittest.mock import Mock, patch
 
 import pytest
@@ -270,7 +270,7 @@ def test_merge_base_preserves_nested_reference_arguments(object_format, ref_form
 
 @pytest.mark.parametrize("object_format", ["sha1", "sha256"])
 @pytest.mark.parametrize("ref_format", ["files", "reftable"])
-@pytest.mark.parametrize("path_type", [Path, PathLikeMock])
+@pytest.mark.parametrize("path_type", [PurePosixPath, PathLikeMock])
 def test_revision_arguments_preserve_pathlike_paths(object_format, ref_format, path_type):
     with fixture_repo("exact-refs.sh", object_format, ref_format, "shadowed") as repo:
         local = repo.head.commit