feat: Add parent trace context propagation for workflow-as-tool hierarchy

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.

api/core/app/apps/workflow/app_generator.py

@@ -147,9 +147,12 @@ class WorkflowAppGenerator(BaseAppGenerator):
 
         inputs: Mapping[str, Any] = args["inputs"]
 
-        extras = {
+        extras: dict[str, Any] = {
             **extract_external_trace_id_from_args(args),
         }
+        parent_trace_context = args.get("_parent_trace_context")
+        if parent_trace_context:
+            extras["parent_trace_context"] = parent_trace_context
         workflow_run_id = str(uuid.uuid4())
         # FIXME (Yeuoly): we need to remove the SKIP_PREPARE_USER_INPUTS_KEY from the args
         # trigger shouldn't prepare user inputs