Add platform-specific skip marker for tests using sys.modules mocking on Linux

Author: Kingshuk-Microsoft

src/tests/backend/test_app.py

@@ -22,6 +22,13 @@
 if src_path not in sys.path:
     sys.path.insert(0, src_path)
 
+# Skip these tests on Linux - they use sys.modules mocking which causes issues
+# See test_app_simple.py for cross-platform tests
+pytestmark = pytest.mark.skipif(
+    platform.system() == 'Linux',
+    reason="sys.modules mocking causes issubclass errors on Linux"
+)
+
 
 class MockUserLanguage(BaseModel):
     """Mock UserLanguage model for testing."""