fix: handle omit type in _remove_not_given() for litellm model (#1893)

Co-authored-by: Kazuhiro Sera <seratch@openai.com>

Author: HareeshBahuleyan

src/agents/extensions/models/litellm_model.py

@@ -500,7 +500,7 @@ def _fix_tool_message_ordering(
         return fixed_messages
 
     def _remove_not_given(self, value: Any) -> Any:
-        if isinstance(value, NotGiven):
+        if value is omit or isinstance(value, NotGiven):
             return None
         return value