Enables distributed tracing for nested workflows across all trace providers
(Langfuse, LangSmith, community providers). When a workflow invokes another
workflow via workflow-as-tool, the child workflow now includes parent context
attributes that allow trace systems to reconstruct the full execution tree.
Changes:
- Add parent_trace_context field to WorkflowTool
- Set parent context in tool node when invoking workflow-as-tool
- Extract and pass parent context through app generator
This is a community enhancement (ungated) that improves distributed tracing
for all users. Parent context includes: trace_id, node_execution_id,
workflow_run_id, and app_id.
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