Add regression test for final_gen_send PERSIS_STOP with empty H_rows

[jlnav]

Summary

Adds a unit-level regression test covering the bug fixed in a5dd111: Manager._send_work_order previously gated the entire PERSIS_STOP/final_gen_send meta-message behind len(work_rows), so when a persistent generator was stopped with zero pending rows (e.g. idle shutdown), the manager sent nothing and the worker hung forever in PersistentSupport.recv().

This was discovered while testing #1737 (libE service mode), but the bug/fix live in core manager code, so this test targets develop directly rather than that feature branch.

Changes

• libensemble/tests/unit_tests/test_manager_main.py: adds a FakeComm helper and test_send_work_order_persis_stop_empty_rows, which exercises Manager._send_work_order directly with a PERSIS_STOP work order and empty H_rows, asserting the meta-message is still sent.

Testing

• Verified the test fails against pre-fix _send_work_order/PersistentSupport.recv() and passes with the fix (already on develop).
• black/flake8 clean.