Docs: add testing and CI documentation

[laura30llorente]

Summary

This PR introduces a complete guide for our testing strategy, quality assurance stack and CI workflow.

It is designed to help current and future developers understand how to write, run and maintain tests within the project.

Changes included

• Testing stack: Documented the use of pytest and its ecosystem (asyncio, mock, cov, dotenv, testdox), along with Ruff and Safety, including direct links to their official PyPI/Documentation pages.
• Project structure & Config: Explained the app/tests/ directory layout, pytest.ini setup and critical conftest.py fixtures.
• Local execution: Added clear instructions for running tests via the Makefile (make test, make test-cov).
• CI pipeline: Documented the GitHub Actions workflow, including a step-by-step guide on how to securely add new environment variables/secrets to the repository.
• Developer guide: Added a practical, step-by-step tutorial on how to mock dependencies using monkeypatch and write new tests for future features.

Closes #29