feat(fusion_accounting_ai): 5 new customer follow-up AI tools

Adds Task 17 tool layer:
- fusion_list_overdue
- fusion_get_partner_followup_detail
- fusion_generate_followup_text
- fusion_send_followup
- fusion_get_partner_risk_score

Tools register through TOOL_DISPATCH and degrade with a clear
error message when fusion_accounting_followup is not installed.
5 TransactionCase tests added (78 total).

Made-with: Cursor

fusion_accounting_followup/tests/__init__.py

@@ -12,3 +12,4 @@ from . import test_fusion_followup_engine
 from . import test_engine_integration
 from . import test_followup_controller
 from . import test_followup_adapter
+from . import test_followup_tools