fix: add reasoning_items to unsupported_delta_fields in streaming serializer

The fast-serialization path dropped delta.reasoning_items by classifying
chunks with empty content and reasoning_items as simple text. Adding the field
to unsupported_delta_fields causes such chunks to fall through to model_dump_json,
preserving the encrypted reasoning state for multi-turn tool loops.

Fixes #38907

litellm/proxy/proxy_server.py

@@ -8268,6 +8268,7 @@ def _fast_serialize_simple_model_response_stream(
         "thinking_blocks",
         "provider_specific_fields",
         "refusal",
+        "reasoning_items",
     )
     if any(getattr(delta, field, None) is not None for field in unsupported_delta_fields):
         return None