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/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting Follow-up',
-    'version': '19.0.1.0.16',
+    'version': '19.0.1.0.17',
     'category': 'Accounting/Accounting',
     'summary': 'AI-augmented customer follow-ups (dunning) for unpaid invoices.',
     'description': """