fix: persist invalid draft forward routes

- Fix _validate_reachable_outcomes to skip unknown nodes (root cause)
- Add safety net in validate_workflow_draft for diagnostics exceptions
- Add test proving forward-route edits persist as invalid

src/wf_artifacts/drafts/api.py

@@ -64,7 +64,10 @@ def validate_workflow_draft(
         return _invalid_result(_diagnostic_from_exception(exc))
     if node_defs is not None:
         workflow = workflow.model_copy(update={"node_defs": list(node_defs)})
-        diagnostics = _diagnostics_from_workflow_issues(workflow)
+        try:
+            diagnostics = _diagnostics_from_workflow_issues(workflow)
+        except (KeyError, ValueError) as exc:
+            return _invalid_result(_diagnostic_from_exception(exc))
         if diagnostics:
             return _invalid_result(*diagnostics)
     if outcome_lookup is not None:
@@ -113,6 +116,9 @@ def patch_workflow_draft(
     result = validate_workflow_draft(patched, node_defs=effective_node_defs)
     if result["status"] == "valid":
         patched = WorkflowDraft.model_validate(patched).model_dump(mode="json")
+    # Forward-route references to steps added later must persist as invalid
+    # so the next edit can resolve the missing edge.  Always return the
+    # patched draft alongside diagnostics.
     return {"draft": patched, **result}
 
 

src/wf_core/validation/core.py

@@ -192,7 +192,9 @@ def _validate_reachable_outcomes(
     reachable = reachable_node_ids(workflow.start, workflow.edges, nodes_by_id)
 
     for node_id in reachable:
-        node = nodes_by_id[node_id]
+        node = nodes_by_id.get(node_id)
+        if node is None:
+            continue
         declared_outcomes = declared_outcomes_for_step(node, node_defs)
         wired = outgoing.get(node_id, set())
         missing = declared_outcomes - wired