testing: add an idempotence / 'clean frame stays clean' invariant test

[JohnnyWilson16]

Problem

CONTRIBUTING.md requires a "does not fire when it shouldn't" test per cleaning step, but there's no shared harness asserting the broad invariant: an already-clean frame should pass through fd.clean unchanged (idempotence / zero Actions).

Expected outcome

A parametrized test that feeds several already-clean frames through fd.clean and asserts the result is unchanged and records no data-modifying Actions.

Relevant files

• New test module under tests/
• Reads the Action model in src/freshdata/report.py

Acceptance criteria

• The test passes on main.
• It would fail if a step mutated a clean column (verify by a local sanity check).
• Fast lane (pytest -m "not online and not large") stays green; coverage gate holds.

Difficulty / skills

🧪 intermediate — pytest, pandas; optional hypothesis for generated clean frames.