feat(studio): 回退/重roll 与 R5 用户确认下一步

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/Studio/studioRunUtils.js

@@ -26,3 +26,18 @@ export function findLastUserMessageIndex(stepMessages = []) {
   }
   return -1;
 }
+
+/** Whether undo is available for the active node state. */
+export function canUndoRun(activeNodeState) {
+  return (activeNodeState?.turnHistory?.length ?? 0) > 0;
+}
+
+/** Whether reroll is available (at least one user message in current step). */
+export function canRerollRun(activeNodeState) {
+  return findLastUserMessageIndex(activeNodeState?.stepMessages) >= 0;
+}
+
+/** Whether the user may advance to the next pipeline node (R5). */
+export function canAdvanceNextStep(activeNodeState) {
+  return Boolean(activeNodeState?.lastDraft);
+}