fix(tenancy): mock billing limit check in store creation unit test

The test was failing because can_create_store() called the billing
module's check_resource_limit() which returned False for the test
merchant (no subscription). Patched the limit check since billing
is not what this unit test exercises.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/tenancy/tests/unit/test_merchant_store_service.py

@@ -2,6 +2,7 @@
 """Unit tests for MerchantStoreService."""
 
 import uuid
+from unittest.mock import patch
 
 import pytest
 
@@ -172,7 +173,8 @@ class TestMerchantStoreServiceCreate:
     def setup_method(self):
         self.service = MerchantStoreService()
 
-    def test_create_store_success(self, db, merchant_owner):
+    @patch.object(MerchantStoreService, "can_create_store", return_value=(True, None))
+    def test_create_store_success(self, _mock_limit, db, merchant_owner):
         """Test successful store creation."""
         unique_id = str(uuid.uuid4())[:8]
         store_data = {