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/auth_fixtures.py

@@ -1,4 +1,10 @@
 # tests/fixtures/auth_fixtures.py
+"""
+Authentication-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 uuid
 
 import pytest
@@ -9,14 +15,14 @@ from models.database.user import User
 
 @pytest.fixture(scope="session")
 def auth_manager():
-    """Create auth manager instance (session scope since it's stateless)"""
+    """Create auth manager instance (session scope since it's stateless)."""
     return AuthManager()
 
 
 @pytest.fixture
 def test_user(db, auth_manager):
-    """Create a test user with unique username"""
-    unique_id = str(uuid.uuid4())[:8]  # Short unique identifier
+    """Create a test user with unique username."""
+    unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("testpass123")
     user = User(
         email=f"test_{unique_id}@example.com",
@@ -28,16 +34,13 @@ def test_user(db, auth_manager):
     db.add(user)
     db.commit()
     db.refresh(user)
-    # Expunge user from session to prevent ResourceWarning about unclosed connections
-    # This detaches the object from the session so it doesn't hold a reference
-    db.expunge(user)
     return user
 
 
 @pytest.fixture
 def test_admin(db, auth_manager):
-    """Create a test admin user with unique username"""
-    unique_id = str(uuid.uuid4())[:8]  # Short unique identifier
+    """Create a test admin user with unique username."""
+    unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("adminpass123")
     admin = User(
         email=f"admin_{unique_id}@example.com",
@@ -49,15 +52,13 @@ def test_admin(db, auth_manager):
     db.add(admin)
     db.commit()
     db.refresh(admin)
-    # Expunge admin from session to prevent ResourceWarning about unclosed connections
-    db.expunge(admin)
     return admin
 
 
 @pytest.fixture
 def another_admin(db, auth_manager):
-    """Create another test admin user for testing admin-to-admin interactions"""
-    unique_id = str(uuid.uuid4())[:8]  # Short unique identifier
+    """Create another test admin user for testing admin-to-admin interactions."""
+    unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("anotheradminpass123")
     admin = User(
         email=f"another_admin_{unique_id}@example.com",
@@ -69,14 +70,12 @@ def another_admin(db, auth_manager):
     db.add(admin)
     db.commit()
     db.refresh(admin)
-    # Expunge admin from session to prevent ResourceWarning about unclosed connections
-    db.expunge(admin)
     return admin
 
 
 @pytest.fixture
 def other_user(db, auth_manager):
-    """Create a different user for testing access controls"""
+    """Create a different user for testing access controls."""
     unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("otherpass123")
     user = User(
@@ -89,8 +88,6 @@ def other_user(db, auth_manager):
     db.add(user)
     db.commit()
     db.refresh(user)
-    # Expunge user from session to prevent ResourceWarning about unclosed connections
-    db.expunge(user)
     return user
 
 
@@ -120,7 +117,7 @@ def admin_headers(client, test_admin):
 
 @pytest.fixture
 def test_vendor_user(db, auth_manager):
-    """Create a test vendor user with unique username"""
+    """Create a test vendor user with unique username."""
     unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("vendorpass123")
     user = User(
@@ -133,7 +130,6 @@ def test_vendor_user(db, auth_manager):
     db.add(user)
     db.commit()
     db.refresh(user)
-    db.expunge(user)
     return user