Improve CI Pipeline: Add Tests & Lint Stages

[ananos]

The current CI may only cover builds or basic checks. We need to extend it with test runs and linting.

Details

We should:

• Ensure CI installs dependencies and runs pytest.
• Polish lint checks (flake8, black --check, isort --check).
• Fail the PR if any step fails.
• Consider caching pip dependencies for faster builds.

This strengthens CI process and prevents broken merges.