feat(fusion_accounting_ai): wire FollowupAdapter fusion paths to engine

- Switch FUSION_MODEL to fusion.followup.engine so adapter mode
  selection matches the new module
- Add list_overdue() with fusion/enterprise/community variants
- Re-route send_followup_via_fusion to engine.send_followup_email
- 4 new TransactionCase tests (73 total)

Existing aging / overdue_invoices adapter methods continue to fall
back to the community implementation.

Made-with: Cursor

fusion_accounting_followup/tests/__init__.py

@@ -11,3 +11,4 @@ from . import test_account_move_line_inherit
 from . import test_fusion_followup_engine
 from . import test_engine_integration
 from . import test_followup_controller
+from . import test_followup_adapter