refactor: reorganize tests into admin and vendor subdirectories

Split integration tests into logical admin/ and vendor/ subdirectories
for better organization. Updated fixture imports and test structure.

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

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

tests/fixtures/auth_fixtures.py

@@ -133,8 +133,11 @@ def test_vendor_user(db, auth_manager):
 
 
 @pytest.fixture
-def vendor_user_headers(client, test_vendor_user):
-    """Get authentication headers for vendor user (uses get_current_vendor_api)"""
+def vendor_user_headers(client, test_vendor_user, test_vendor_with_vendor_user):
+    """Get authentication headers for vendor user (uses get_current_vendor_api).
+
+    Depends on test_vendor_with_vendor_user to ensure VendorUser association exists.
+    """
     response = client.post(
         "/api/v1/vendor/auth/login",
         json={"email_or_username": test_vendor_user.username, "password": "vendorpass123"},