[Fix] Add flaky marker to test_pdf_handling integration test

The test makes real API calls to OpenAI's o1 model which can crash
pytest-xdist workers in CI, especially the async variant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/llm_translation/base_llm_unit_tests.py

@@ -347,6 +347,7 @@ class BaseLLMChatTest(ABC):
         assert response is not None
         print(f"response={response}")
 
+    @pytest.mark.flaky(retries=3, delay=1)
     @pytest.mark.parametrize("sync_mode", [True, False])
     @pytest.mark.asyncio
     async def test_pdf_handling(self, pdf_messages, sync_mode):