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

api/core/tools/workflow_as_tool/tool.py

@@ -50,6 +50,7 @@ class WorkflowTool(Tool):
         self.workflow_call_depth = workflow_call_depth
         self.label = label
         self._latest_usage = LLMUsage.empty_usage()
+        self.parent_trace_context: dict[str, str] | None = None
 
         super().__init__(entity=entity, runtime=runtime)
 
@@ -90,11 +91,15 @@ class WorkflowTool(Tool):
 
         self._latest_usage = LLMUsage.empty_usage()
 
+        args: dict[str, Any] = {"inputs": tool_parameters, "files": files}
+        if self.parent_trace_context:
+            args["_parent_trace_context"] = self.parent_trace_context
+
         result = generator.generate(
             app_model=app,
             workflow=workflow,
             user=user,
-            args={"inputs": tool_parameters, "files": files},
+            args=args,
             invoke_from=self.runtime.invoke_from,
             streaming=False,
             call_depth=self.workflow_call_depth + 1,