fix(frontend): treat any task tool error as a terminal subtask failure

The subtask card status machine matched only three English prefixes (`Task
Succeeded. Result:`, `Task failed.`, `Task timed out`). Anything else fell
through to `in_progress`, so a `task` tool error wrapped by
`ToolErrorHandlingMiddleware` (`Error: Tool 'task' failed ...`) left the card
spinning forever even after the run had ended.

Extract the prefix logic into `parseSubtaskResult` and recognise any leading
`Error:` token as a terminal failure. The extracted function is unit-tested
against the legacy prefixes plus the `AsyncCallbackManager` regression
captured in the upstream issue.

Refs: bytedance/deer-flow#3107 (BUG-007)

frontend/src/components/workspace/messages/message-list.tsx

@@ -27,6 +27,7 @@ import {
 import { useRehypeSplitWordsIntoSpans } from "@/core/rehype";
 import type { Subtask } from "@/core/tasks";
 import { useUpdateSubtask } from "@/core/tasks/context";
+import { parseSubtaskResult } from "@/core/tasks/subtask-result";
 import type { AgentThreadState } from "@/core/threads";
 import { cn } from "@/lib/utils";
 
@@ -359,33 +360,10 @@ export function MessageList({
               } else if (message.type === "tool") {
                 const taskId = message.tool_call_id;
                 if (taskId) {
-                  const result = extractTextFromMessage(message);
-                  if (result.startsWith("Task Succeeded. Result:")) {
-                    updateSubtask({
-                      id: taskId,
-                      status: "completed",
-                      result: result
-                        .split("Task Succeeded. Result:")[1]
-                        ?.trim(),
-                    });
-                  } else if (result.startsWith("Task failed.")) {
-                    updateSubtask({
-                      id: taskId,
-                      status: "failed",
-                      error: result.split("Task failed.")[1]?.trim(),
-                    });
-                  } else if (result.startsWith("Task timed out")) {
-                    updateSubtask({
-                      id: taskId,
-                      status: "failed",
-                      error: result,
-                    });
-                  } else {
-                    updateSubtask({
-                      id: taskId,
-                      status: "in_progress",
-                    });
-                  }
+                  const parsed = parseSubtaskResult(
+                    extractTextFromMessage(message),
+                  );
+                  updateSubtask({ id: taskId, ...parsed });
                 }
               }
             }