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,