Add CI tests for python backend

**Is your feature request related to a problem? Please describe.**
Currently Stave repo only has a single [backend test](https://github.com/asyml/stave/blob/master/simple-backend/test.py) and it is not integrated into the [GitHub workflow](https://github.com/asyml/stave/blob/master/.github/workflows/main.yml).

**Describe the solution you'd like**
We want a comprehensive backend test that consists of unit tests validating all the major functionalities of Stave backend. These tests should be included into CI workflow such that every PR will be checked upon each new commit.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

Start by inspecting simple-backend/test.py to understand the existing backend test, then read .github/workflows/main.yml to see how checks are currently configured. Map the backend’s major functionalities to unit-test coverage and update the workflow so the backend tests run for every pull request and commit; done means comprehensive tests pass in CI.

Written by the indexing model from the issue text.