Add Testing Documentation to the Project

Description:

The purpose of this issue is to prepare a comprehensive testing documentation file to guide contributors in writing, organising, and running tests consistently. The document should include testing principles, test types, writing guidelines, and CI integration practices.

Tasks:
- [ ] Create a new file TESTING.md in the project root.

- [ ] Add sections for:

- Testing Principles
- Test Types (Unit, Integration, Edge Case, Regression)
- Writing Tests (Structure and Checklist)
- Running Tests
- Tools and Frameworks
- Continuous Integration

- [ ] Verify all instructions are accurate and applicable to the project.

- [ ] Add example configurations for CI and test coverage.

- [ ] Submit the document for review.

References:

- Refer to the draft document prepared in the `TESTING.md`