Ran confirmed LangGraphAGUIAgent is the only relevant path going forward.
Removes LangGraphAgent, _StreamingStateExtractor, CopilotKitConfig, and
langgraph_default_merge_state from sdk-python (the AG-UI base class
provides this now). Also removes the use_function_call=True code path
from copilotkit_messages_to_langchain (no callers, function_call format
is obsolete).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>