[FIX]: suppress ty invalid-assignment on intentional frozen-config del test

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 0cbb94b4-d5d2-4be8-9e5a-ba0e28562070

Author: nina-msft

tests/unit/core/test_llm.py

@@ -60,7 +60,7 @@ def test_cannot_set_field(self):
     def test_cannot_delete_field(self):
         cfg = LLMConfig(model="gpt-4o", endpoint="https://api.example.com")
         with pytest.raises(AttributeError):
-            del cfg.model
+            del cfg.model  # ty: ignore[invalid-assignment]
 
 
 class TestLLMConfigEquality: