fix(proxy): ignore None-valued extras in the streaming fast path guard

The stream wrapper sets citations=None on every OpenAI-style chunk, so the
previous guard sent almost every chunk to the slow path. exclude_none drops
None extras on the slow path too, so only non-None extras need it.

litellm/proxy/proxy_server.py

@@ -9568,9 +9568,11 @@ def _fast_serialize_simple_model_response_stream(
     # Fields set on the chunk outside its schema (e.g. by a streaming iterator
     # hook) live in ``model_extra``. The payload below only writes
     # ``service_tier``, so any other extra would be silently dropped here
-    # while the canonical serializer keeps it.
+    # while the canonical serializer keeps it. None-valued extras (e.g. the
+    # ``citations=None`` the stream wrapper sets on every chunk) are dropped by
+    # ``exclude_none`` there too, so they must not force the slow path.
     model_extra: Final = getattr(chunk, "model_extra", None)
-    if model_extra and any(key != "service_tier" for key in model_extra):
+    if model_extra and any(key != "service_tier" and value is not None for key, value in model_extra.items()):
         return None
 
     choices: Final = getattr(chunk, "choices", None)

tests/unit/proxy/proxy_server/test_streaming_helpers.py

@@ -558,6 +558,17 @@ def test_fast_serialize_simple_model_response_stream_with_extra_field_returns_no
     assert _fast_serialize_simple_model_response_stream(chunk) is None
 
 
+def test_fast_serialize_simple_model_response_stream_with_none_extra_uses_fast_path():
+    """The stream wrapper sets ``citations=None`` on every OpenAI-style chunk.
+    ``exclude_none`` drops it on the slow path too, so it must not disable
+    the fast path."""
+    chunk = _simple_chunk()
+    chunk.citations = None
+    fast = _fast_serialize_simple_model_response_stream(chunk)
+    assert fast is not None
+    assert "citations" not in json.loads(fast)
+
+
 def test_serialize_streaming_chunk_keeps_extra_field():
     chunk = _simple_chunk()
     chunk.gateway_metadata = {"served_by": "fallback-model"}