fix(frontend): parse Task cancelled and polling timed out as terminal

Address @ShenAC-SAC's BUG-007 review on #3131. `task_tool.py` actually
emits five terminal strings:

- `Task Succeeded. Result: …`
- `Task failed. …`
- `Task timed out. …`
- `Task cancelled by user.`               ← previously matched none
- `Task polling timed out after N minutes …` ← previously matched none

The previous cut handled three; the last two fell through to the
"unknown content" branch and pushed the subtask card back to
`in_progress` even though the backend had already reached a terminal
state. Add explicit matches plus regression tests for both. The
`in_progress` fallback is now reserved for genuinely unrecognised
output (i.e. contract drift), as documented.

Refs: bytedance/deer-flow#3107 (BUG-007), bytedance/deer-flow#3131 review

frontend/src/core/tasks/subtask-result.ts

@@ -23,6 +23,8 @@ export interface SubtaskResultUpdate {
 export const SUCCESS_PREFIX = "Task Succeeded. Result:";
 export const FAILURE_PREFIX = "Task failed.";
 export const TIMEOUT_PREFIX = "Task timed out";
+export const CANCELLED_PREFIX = "Task cancelled by user.";
+export const POLLING_TIMEOUT_PREFIX = "Task polling timed out";
 export const ERROR_WRAPPER_PATTERN = /^Error\b/i;
 
 /**
@@ -62,6 +64,14 @@ export function parseSubtaskResult(text: string): SubtaskResultUpdate {
     return { status: "failed", error: trimmed };
   }
 
+  if (trimmed.startsWith(CANCELLED_PREFIX)) {
+    return { status: "failed", error: trimmed };
+  }
+
+  if (trimmed.startsWith(POLLING_TIMEOUT_PREFIX)) {
+    return { status: "failed", error: trimmed };
+  }
+
   // ToolErrorHandlingMiddleware-style wrapper, or any other terminal error
   // signal the backend forwards to the lead agent.
   if (ERROR_WRAPPER_PATTERN.test(trimmed)) {

frontend/tests/unit/core/tasks/subtask-result.test.ts

@@ -25,6 +25,25 @@ describe("parseSubtaskResult", () => {
     expect(parsed.error).toBe("Task timed out after 900s");
   });
 
+  it("recognises the cancelled-by-user prefix", () => {
+    // bytedance/deer-flow#3131 review: this is one of the five terminal
+    // strings task_tool.py actually emits — the previous cut treated it as
+    // unrecognised content and pushed the card back to in_progress.
+    const parsed = parseSubtaskResult("Task cancelled by user.");
+    expect(parsed.status).toBe("failed");
+    expect(parsed.error).toBe("Task cancelled by user.");
+  });
+
+  it("recognises the polling-timed-out prefix", () => {
+    // Emitted by task_tool when the background polling loop runs out of
+    // budget waiting for the subagent to reach a terminal state.
+    const parsed = parseSubtaskResult(
+      "Task polling timed out after 15 minutes. This may indicate the background task is stuck. Status: RUNNING",
+    );
+    expect(parsed.status).toBe("failed");
+    expect(parsed.error).toContain("polling timed out");
+  });
+
   it("treats middleware-wrapped tool errors as terminal failures", () => {
     // bytedance/deer-flow issue #3107 BUG-007: the parent-visible ToolMessage
     // produced by ToolErrorHandlingMiddleware never matches the three legacy