Add more tests for chat functionality

[apkostka]

[yangm2]

To get an idea about missing tests

1. change dir to backend
2. run

   uv run pytest --cov tenantfirstaid --cov-report html --cov-branch

3. open htmlcov/index.html in a browser
4. click on an individual file to see missing line and branch coverage

You should see something like ...

and ...