Align standard provider empty and external_task tests with expected paths

[keith991001]

Resolves two OVERLOOKED_TESTS entries for the standard provider, following the per-provider batching convention (same shape as #73111 / #73153 / #73229).

What changed

sensors/test_external_task.py — the coverage already existed (2400+ lines) but under the legacy name test_external_task_sensor.py, so the guard list kept treating the mirror-path test as missing (the stale entry was never caught because of the added_test_files type mismatch that #71985 fixes). Renamed via git mv to mirror the source module external_task.py, matching the earlier branch/latest_only alignment in #73229.

While moving, added a TestExternalDagLink class — the ExternalDagLink operator link had no dedicated coverage: its name, and the exact dag-run URL it builds, asserted for both operators that declare it (ExternalTaskSensor and ExternalTaskMarker).

operators/test_empty.py — new file. EmptyOperator is 39 lines of source but carries a scheduler-facing contract worth pinning:

• execute is a no-op returning None;
• inherits_from_empty_operator is True on class and instance;
• the scheduler short-circuit contract via TaskInstance.is_task_schedulable (3.2+ guarded): a trivial EmptyOperator is marked success without being scheduled, while on_execute_callback / on_success_callback / outlets / inlets each opt the task back into real scheduling.

Both entries removed from OVERLOOKED_TESTS in test_project_structure.py.

Verification

• pytest providers/standard/tests/unit/standard/operators/test_empty.py → 7 passed
• pytest providers/standard/tests/unit/standard/sensors/test_external_task.py → 42 passed, 65 skipped (pre-existing version-gated skips, unchanged by the move)
• pytest airflow-core/tests/unit/always/test_project_structure.py → 10 passed, 1 xfailed
• local providers mypy and prek run --from-ref main clean

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