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/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting Follow-up',
-    'version': '19.0.1.0.15',
+    'version': '19.0.1.0.16',
     'category': 'Accounting/Accounting',
     'summary': 'AI-augmented customer follow-ups (dunning) for unpaid invoices.',
     'description': """

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

fusion_accounting_followup/tests/test_followup_adapter.py

@@ -0,0 +1,42 @@
+"""FollowupAdapter wiring tests — engine paths."""
+
+from odoo.tests import tagged
+from odoo.tests.common import TransactionCase
+
+from odoo.addons.fusion_accounting_ai.services.data_adapters.followup import (
+    FollowupAdapter,
+)
+
+
+@tagged('post_install', '-at_install')
+class TestFollowupAdapter(TransactionCase):
+
+    def setUp(self):
+        super().setUp()
+        self.adapter = FollowupAdapter(self.env)
+
+    def test_list_overdue_via_fusion_returns_dict(self):
+        result = self.adapter.list_overdue_via_fusion(
+            company_id=self.env.company.id,
+        )
+        self.assertIn('partners', result)
+        self.assertIn('total', result)
+        self.assertIn('count', result)
+
+    def test_list_overdue_via_community_returns_error(self):
+        result = self.adapter.list_overdue_via_community()
+        self.assertIn('error', result)
+
+    def test_send_followup_via_fusion_no_overdue(self):
+        partner = self.env['res.partner'].create({'name': 'AdapterTest'})
+        result = self.adapter.send_followup_via_fusion(
+            partner_id=partner.id, force=True,
+        )
+        self.assertIn(
+            result.get('status', ''),
+            ('no_action', 'no_overdue', 'sent', 'manual_review'),
+        )
+
+    def test_send_followup_via_community_returns_error(self):
+        result = self.adapter.send_followup_via_community(partner_id=1)
+        self.assertIn('error', result)