test: clean up read-only submodule object files on Windows

The Windows Python 3.15 CI job failed to set up twelve missing-submodule
cases because `shutil.rmtree()` cannot remove read-only loose Git objects.
The fixture deliberately removes retained metadata to model an absent
submodule, so use the existing `git.util.rmtree()` helper, which clears
read-only attributes when retrying Windows deletions.

All twelve affected cases pass locally, along with Ruff lint and format
checks. Production behavior and test coverage are unchanged.

Author: codex

test/test_submodule_no_fetch.py

@@ -3,13 +3,13 @@
 
 from functools import partial
 from pathlib import Path
-import shutil
 from unittest import mock
 
 import pytest
 
 from git import Git, Remote, Repo, RootModule, Submodule
 from git.exc import GitCommandError
+from git.util import rmtree
 
 
 def _commit_file(repo, content):
@@ -60,7 +60,7 @@ def uninitialized_submodule(request, local_submodule):
     else:
         submodule.remove(configuration=False, force=True)
         # Removal retains recoverable metadata; remove it explicitly for this fixture.
-        shutil.rmtree(metadata)
+        rmtree(metadata)
     assert not submodule.module_exists()
     assert metadata.is_dir() == (request.param == "deinitialized")
     return submodule, metadata