fix: handle files on different drives when finding the common root - #5939

Summary

os.path.commonpath() raises ValueError when file paths are on different Windows drives. find_common_root() only catches OSError, so initializing a coder with those files can crash instead of using its existing current-directory fallback.

Catch ValueError alongside OSError. Add a regression test using the real ntpath.commonpath implementation, so the Windows case can also be exercised on other platforms.

Fixes #3704.

Validation

• Against the original exception handler: 1 failed, with ValueError: Paths don't have the same drive.
• python -m pytest tests/basic/test_find_common_root.py -q: 1 passed.
• Repository-configured pre-commit hooks on both changed files: isort, Black, flake8 and codespell passed.
• git diff --check passed.

The full suite and live model integrations were not run. This change was prepared with AI assistance and checked with the regression test above.