feat: route LangSmith traces from thread streams (#2745)

## Description
Expose LangSmith tracer settings on JavaScript thread-stream run starts
and forward them through the protocol.

## Release Note
JavaScript thread streams can route traces to an additional LangSmith
project per run.

## Test Plan
- [x] Verify run-start commands preserve tracing settings

## Related PRs
- langchain-ai/agent-protocol#95
- langchain-ai/langgraph#8723
- langchain-ai/langgraph-api#4033

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Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Christian Bromann <git@bromann.dev>