feat(langgraph): add forwarded props to langgraph runtime context

[shell-nlp]

Background

Issue #1815 was addressed by #1826, which updated
LangGraphAgent.get_stream_kwargs() to support forwarding a context
argument to LangGraph, including LangGraph implementations whose
astream_events() signature accepts **kwargs.

However, #1826 only added the helper-level support. Neither the normal
streaming path nor the regenerate/time-travel path supplied request-level
values to get_stream_kwargs(context=...). Consequently, agent-side
forwardedProps could not actually reach LangGraph's
astream_events(..., context=...) runtime context.

This PR is a small follow-up to #1826 that wires the missing runtime call path
described above.

Refs #1815
Follow-up to #1826

Summary

This is a small, backwards-compatible adapter plumbing change, not a major
feature or protocol change.

It forwards agent-facing forwardedProps into LangGraph's runtime context
when calling graph.astream_events(...).

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