1. 1. Summary

Fix `get_copybara_source_mirror_url()` so the Linux container path is converted to a file URI with POSIX semantics on every host OS. Add regression coverage in `buildscripts/tests/test_sync_repo_with_copybara.py`.

1. 1. Operational benefit

Windows compile-required builds no longer fail while preparing the local Copybara source mirror.

1. 1. Safety and compatibility

The change only affects conversion of the fixed container-local path and preserves the existing Linux URI.

1. 1. Testing

• `bazel run format`
• `python3-venv/bin/python -m py_compile buildscripts/copybara/sync_repo_with_copybara.py buildscripts/tests/test_sync_repo_with_copybara.py`