Add missing unit tests for common.compat lineage entities - #72506

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Note on the red Compat 2.11.1:P3.10 lane: the new test_entities.py module itself passed all 21 tests there — the failure is providers/openlineage/tests/unit/openlineage/plugins/test_execution.py::test_short_stalled_task_extraction_with_low_execution_is_killed_by_ol_timeout, a timing-sensitive test unrelated to this PR (which only adds a test module under common/compat). A re-run should clear it.

Drafted-by: Claude Code (Fable 5) (no human review before posting)

@potiuk
gentle reminder— this PR is approved, rebased onto latest main (the conflict with #72509 is resolved), and CI is fully green. Since OVERLOOKED_TESTS in test_project_structure.py is a high-churn file, it tends to conflict again if it sits — would appreciate a merge when you have a moment. Thanks!