Add v2.0 fixtures tests, mock CLI utility, and tests README

[guillermodotn]

• Add test_compose_v2.py with structural validation tests for v2.0 compose fixtures.
• Add mock_cli.py standalone tool for developing and testing CLI output behaviour.
• Add tests/README.md.
• Update uv.lock with new dependencies.

[lubomir]

Could you please elaborate on what is the point of the mock cli? It is not used by any test, and doesn't share much code with the real tools either.

[guillermodotn]

Could you please elaborate on what is the point of the mock cli? It is not used by any test, and doesn't share much code with the real tools either.

It's a development tool for iterating on the CLI output formatting. You run it as a script to see how the progress bars, per-artifact log lines, ... look without needing a real compose neither network services.