test: add Hypothesis tests for rich_observers and runner modules

- Create test_rich_observers.py with unit and property-based tests for
  SimpleProgressObserver and RichDashboardObserver (100% coverage)
- Add RunnerStats.merge() tests with stateful testing
- Add ValidationRunner context manager tests
- Add parallel processing tests for _run_parallel and _validate_chunk
- Add property-based tests for RunnerStats invariants
- Coverage increased: rich_observers.py 16% -> 100%, runner.py 64% -> 100%

Author: jreakin