[Bugfix] [Frontend] Cleanup gpt-oss non-streaming chat tool calls (#25514)

Signed-off-by: Ben Browning <bbrownin@redhat.com>

tests/entrypoints/openai/test_serving_chat.py

@@ -194,6 +194,7 @@ async def test_gpt_oss_multi_turn_chat(gptoss_client: OpenAI,
     assert tc.function is not None and tc.function.name == "get_current_weather"
     args1 = tc.function.arguments
     assert args1 is not None and len(args1) > 0
+    assert not first_msg.content
 
     messages.append({"role": "assistant", "content": args1})
     messages.append({