test(fusion_accounting_followup): engine integration tests for full lifecycle

End-to-end flows over a real posted receivable line: aging discovery,
level resolution, send-with-cache reuse, pause+force override, and
audit history growth. Adds ignore_pause kwarg to compute_followup_level
so force=True in send_followup_email reaches level resolution.

Made-with: Cursor

fusion_accounting_followup/tests/__init__.py

@@ -9,3 +9,4 @@ from . import test_fusion_followup_text_cache
 from . import test_res_partner_inherit
 from . import test_account_move_line_inherit
 from . import test_fusion_followup_engine
+from . import test_engine_integration