refactor: update test imports to use module locations

Update all test files to import from canonical module locations:
- Integration tests: orders, inventory, messages, invoices
- Unit tests: services and models
- Fixtures: customer, vendor, message fixtures

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/models/schema/test_customer.py

@@ -4,7 +4,7 @@
 import pytest
 from pydantic import ValidationError
 
-from models.schema.customer import (
+from app.modules.customers.schemas import (
     CustomerAddressCreate,
     CustomerAddressResponse,
     CustomerAddressUpdate,

tests/unit/models/schema/test_inventory.py

@@ -4,7 +4,7 @@
 import pytest
 from pydantic import ValidationError
 
-from models.schema.inventory import (
+from app.modules.inventory.schemas import (
     InventoryAdjust,
     InventoryCreate,
     InventoryLocationResponse,

tests/unit/models/schema/test_order.py

@@ -6,7 +6,7 @@ from datetime import datetime, timezone
 import pytest
 from pydantic import ValidationError
 
-from models.schema.order import (
+from app.modules.orders.schemas import (
     AddressSnapshot,
     AddressSnapshotResponse,
     CustomerSnapshot,
@@ -573,7 +573,7 @@ class TestOrderItemResponseSchema:
         assert response.has_unresolved_exception is False
 
         # Pending exception
-        from models.schema.order import OrderItemExceptionBrief
+        from app.modules.orders.schemas import OrderItemExceptionBrief
         pending_exc = OrderItemExceptionBrief(
             id=1, original_gtin="123", original_product_name="Test",
             exception_type="product_not_found", status="pending",