Nexent legacy Python unit tests

Paths below are relative to the repository root.

These tests are a legacy suite kept separate from the requirement-driven D1-D5 system. Do not assign formal D1-D5 case IDs to them, add them to test/manifests/d1-d5.yaml, or count them in the generated functional baseline. Use nexent-test-assets for new D1-D5 work.

1. Identify the unit and behavior. Inspect neighboring tests, test/conftest.py, and test/pytest.ini before changing fixture/import setup.
2. Use pytest exclusively, pytest assertions, fixtures, and pytest-mock. Files/functions start with test_; test classes start with Test. Keep files below 500 lines or split by feature using test_<module>_<feature>.py; split package directories include __init__.py.
3. Import the unit and necessary test helpers. Mock collaborators rather than exercising external interfaces/clients/services. Patch the fully qualified lookup site determined from actual imports, not the dependency's definition module.

For example, if backend.services.example_service imports fetch using from provider import fetch, patch backend.services.example_service.fetch. If the runtime loads the module as services.example_service, use that actual path. Do not copy an unrelated service path from an example.

4. Isolate external I/O and APIs. Reset mutable state with fixtures; use autouse=True when every test in a scope requires it. Do not mock away the behavior being asserted.
5. Cover changed success/error flows and boundaries with specific observable assertions. Use side_effect for collaborator errors, @pytest.mark.parametrize for variants, and @pytest.mark.asyncio for async tests. Async collaborators need awaitable mocks.
6. Order imports as standard library, third-party, then project; write comments/docstrings in English.
7. Run narrow tests from the root with a working backend environment, such as pytest test/backend/apps/test_agent_app.py -v. Broaden when needed; python test/run_all_test.py is the existing broad runner.

Report commands, results, and environment failures. Unit-test isolation does not replace live-service functional checks or real-model acceptance.