[jay23606]

Summary

Normalize Windows path separators before filtering files by client in PySUS.query, so FTP files are not dropped on Windows.

Fixes #372

Validation

• python -m pytest pysus/tests/api/test_client.py -q — 51 passed.
• Regression gate — the focused test failed on both clean-base worktrees and passed on both fix worktrees.
• Black, isort, Flake8, and git diff --check passed.