feat(billing): subscription creation handler (sale.order is_subscription)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_centralize_billing/tests/test_api.py

@@ -45,3 +45,19 @@ class TestApiHandlers(TransactionCase):
         cat = self.service._api_catalog()
         codes = [c['plan_code'] for c in cat['charges']]
         self.assertIn('maps-business', codes)
+
+    def test_api_create_subscription(self):
+        self.service._api_upsert_customer({'external_id': 'client-9', 'name': 'Globex'})
+        product = self.env['product.product'].sudo().create(
+            {'name': 'Maps Business', 'type': 'service', 'recurring_invoice': True,
+             'list_price': 249.0})
+        res = self.service._api_create_subscription({
+            'external_customer_id': 'client-9',
+            'plan_id': self.plan.id,
+            'lines': [{'product_id': product.id, 'quantity': 1}],
+        })
+        self.assertEqual(res['status'], 'ok')
+        sub = self.env['sale.order'].browse(res['subscription_id'])
+        self.assertTrue(sub.is_subscription)
+        self.assertEqual(sub.plan_id, self.plan)
+        self.assertEqual(sub.subscription_state, '3_progress')