tests: refactor task-list-tests (direct functional calls + happy path) (#305)

# Description

Fixes #300 

- [x] Migration test already covered in `tasks_migration_test.py`
- [x] happy path test of the python endpoint by callin the API
- [x] have tests on input validation that call the Python API.
Specifically for input validation that are handled through
FastAPI+Pydantic's function type annotation
- [x] the other tests are direct functional tests which test the
behavior of the functions, including correct processing of parameters
and raising of errors, through calling the Python function which
implements the endpoint correctly, as suggested.

Author: saathviksheerla