[Refactor] Move MM item count validation outside of processor (#33396)

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

tests/multimodal/test_processing.py

@@ -921,7 +921,7 @@ def test_limit_mm_per_prompt_dummy(model_id, limit, num_supported, is_valid):
     )
 
     processor = MULTIMODAL_REGISTRY.create_processor(model_config)
-    processor._supported_mm_limits = {"image": num_supported}
+    processor.info.get_supported_mm_limits = lambda: {"image": num_supported}
 
     exc_ctx = nullcontext() if is_valid else pytest.raises(ValueError, match="At most")