refactor: remove db.expunge() anti-pattern from test fixtures

Remove db.expunge() calls that were causing DetachedInstanceError
when accessing lazy-loaded relationships in tests.

Changes:
- conftest.py: Add documentation about fixture best practices
- auth_fixtures: Remove expunge, keep objects attached to session
- customer_fixtures: Remove expunge, add proper relationship loading
- vendor_fixtures: Remove expunge, add test_company and other_company
  fixtures for proper company-vendor relationship setup
- marketplace_import_job_fixtures: Remove expunge calls
- marketplace_product_fixtures: Remove expunge calls

The db fixture already provides test isolation by dropping/recreating
tables after each test, so expunge is unnecessary and harmful.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/fixtures/customer_fixtures.py

@@ -1,12 +1,19 @@
 # tests/fixtures/customer_fixtures.py
+"""
+Customer-related test fixtures.
+
+Note: Fixtures should NOT use db.expunge() as it breaks lazy loading.
+See tests/conftest.py for details on fixture best practices.
+"""
 import pytest
 
 from models.database.customer import Customer, CustomerAddress
+from models.database.order import Order
 
 
 @pytest.fixture
 def test_customer(db, test_vendor):
-    """Create a test customer"""
+    """Create a test customer."""
     customer = Customer(
         vendor_id=test_vendor.id,
         email="testcustomer@example.com",
@@ -19,14 +26,12 @@ def test_customer(db, test_vendor):
     db.add(customer)
     db.commit()
     db.refresh(customer)
-    # Expunge customer from session to prevent ResourceWarning about unclosed connections
-    db.expunge(customer)
     return customer
 
 
 @pytest.fixture
 def test_customer_address(db, test_vendor, test_customer):
-    """Create a test customer address"""
+    """Create a test customer address."""
     address = CustomerAddress(
         vendor_id=test_vendor.id,
         customer_id=test_customer.id,
@@ -42,6 +47,24 @@ def test_customer_address(db, test_vendor, test_customer):
     db.add(address)
     db.commit()
     db.refresh(address)
-    # Expunge address from session to prevent ResourceWarning about unclosed connections
-    db.expunge(address)
     return address
+
+
+@pytest.fixture
+def test_order(db, test_vendor, test_customer, test_customer_address):
+    """Create a test order."""
+    order = Order(
+        vendor_id=test_vendor.id,
+        customer_id=test_customer.id,
+        order_number="TEST-ORD-001",
+        status="pending",
+        subtotal=99.99,
+        total_amount=99.99,
+        currency="EUR",
+        shipping_address_id=test_customer_address.id,
+        billing_address_id=test_customer_address.id,
+    )
+    db.add(order)
+    db.commit()
+    db.refresh(order)
+    return order