[Bugfix] Fix wrong mock attribute (#29704)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

Author: DarkLight1337

tests/entrypoints/openai/test_serving_chat.py

@@ -399,7 +399,7 @@ async def _fake_process_inputs(
 @dataclass
 class MockEngine:
     model_config: MockModelConfig = field(default_factory=MockModelConfig)
-    processor: MagicMock = field(default_factory=MagicMock)
+    input_processor: MagicMock = field(default_factory=MagicMock)
     io_processor: MagicMock = field(default_factory=MagicMock)