[codex] Isolate apps review validation imports

[tianmind-studio]

Summary

• Temporarily stub python_multipart while importing routers.apps in test_apps_review_reply_validation.py so the test can collect in lightweight Windows backend environments without the full multipart package installed.
• Snapshot and restore pre-existing stubbed modules around the heavy router import instead of clearing all matching sys.modules entries globally.
• Keep the test focused on reply_to_review validation while avoiding cross-test collection pollution.

Root cause

routers.apps defines Form / File endpoints at import time. FastAPI validates that python-multipart is installed while building those routes, so this lightweight Windows backend venv failed before the review-reply assertions could collect:

RuntimeError: Form data requires "python-multipart" to be installed.

After fixing that collection issue, running this file with another stub-heavy test exposed an existing cleanup problem: the test removed stubs installed by other collected test modules. Restoring a snapshot keeps the import isolation local to this file.

Validation

• python -m pytest backend/tests/unit/test_apps_review_reply_validation.py --collect-only -q --tb=short -> 4 tests collected
• python -m pytest backend/tests/unit/test_apps_review_reply_validation.py -q --tb=short -> 4 passed
• python -m pytest backend/tests/unit/test_action_item_date_validation.py backend/tests/unit/test_apps_review_reply_validation.py -q --tb=short -> 30 passed
• python -m pytest backend/tests/unit/test_apps_review_reply_validation.py backend/tests/unit/test_action_item_date_validation.py -q --tb=short -> 30 passed
• black --line-length 120 --skip-string-normalization backend/tests/unit/test_apps_review_reply_validation.py
• python -m py_compile backend/tests/unit/test_apps_review_reply_validation.py
• git diff --check
• scripts/pre-commit