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/conftest.py

@@ -1,4 +1,23 @@
 # tests/conftest.py - Updated main conftest with core fixtures only
+"""
+Core pytest configuration and fixtures.
+
+IMPORTANT - Fixture Best Practices:
+===================================
+1. DO NOT use db.expunge() on fixtures - it detaches objects from the session
+   and breaks lazy loading of relationships (e.g., product.marketplace_product).
+
+2. Test isolation is achieved through the db fixture which drops and recreates
+   all tables after each test - no need to manually detach objects.
+
+3. If you need to ensure an object has fresh data, use db.refresh(obj) instead
+   of expunge/re-query patterns.
+
+4. The session uses expire_on_commit=False to prevent objects from being expired
+   after commits, which helps with relationship access across operations.
+
+See docs/testing/testing-guide.md for comprehensive testing documentation.
+"""
 import pytest
 from fastapi.testclient import TestClient
 from sqlalchemy import create_engine
@@ -16,7 +35,7 @@ SQLALCHEMY_TEST_DATABASE_URL = "sqlite:///:memory:"
 
 @pytest.fixture(scope="session")
 def engine():
-    """Create test database engine"""
+    """Create test database engine."""
     return create_engine(
         SQLALCHEMY_TEST_DATABASE_URL,
         connect_args={"check_same_thread": False},
@@ -27,13 +46,34 @@ def engine():
 
 @pytest.fixture(scope="session")
 def testing_session_local(engine):
-    """Create session factory for tests"""
-    return sessionmaker(autocommit=False, autoflush=False, bind=engine)
+    """
+    Create session factory for tests.
+
+    Uses expire_on_commit=False to prevent objects from being expired after
+    commits. This allows fixtures to remain usable after database operations
+    without needing to refresh or re-query them.
+    """
+    return sessionmaker(
+        autocommit=False,
+        autoflush=False,
+        bind=engine,
+        expire_on_commit=False,  # Prevents lazy-load issues after commits
+    )
 
 
 @pytest.fixture(scope="function")
 def db(engine, testing_session_local):
-    """Create a database session for direct database operations"""
+    """
+    Create a database session for each test function.
+
+    Provides test isolation by:
+    - Creating fresh tables before each test
+    - Dropping all tables after each test completes
+
+    Note: Fixtures should NOT use db.expunge() as this detaches objects
+    from the session and breaks lazy loading. The table drop/create cycle
+    provides sufficient isolation between tests.
+    """
     # Create all tables
     Base.metadata.create_all(bind=engine)