test(refs): use POSIX paths for Git reference names

<!-- agent -->
Restore Windows CI coverage for `PathLike` revision arguments. The test
introduced in `6de90f1c` uses `Path("refs/heads/main")`, which emits
backslashes on Windows. Those are invalid Git reference names, so resolving
`SymbolicReference` fails during ref-name validation in all four
files/reftable and SHA-1/SHA-256 variants. The same failures were already
present in PR #2336, before the unrelated `patch_env()` change in PR #2340.

Use `PurePosixPath` for the test's Git reference names, which use forward
slashes on every platform. It still implements `os.PathLike`, preserving
coverage through `SymbolicReference` and direct and nested `merge_base()`
arguments alongside `PathLikeMock`. Keep native `Path` for filesystem
operations elsewhere in the module and preserve production validation.

Assisted-by: GPT 6.1 Sol
Co-authored-by: GPT 6.1 Sol <codex@openai.com>

Author: Byron

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