fix(tasks): stop TaskRunner refresh loop for FAILED task without result

A FAILED task with result == null (e.g. Mapper LLM preview with no provider
configured) made the selectedTask subscriber call refreshTaskDetails, whose
selectedTask.set re-fired the subscriber, looping forever and reopening the
WebSocket / refetching logs each pass. Re-initialize only when the task id
changes and run the detail refresh at most once per task id.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/components/tasks/TaskRunner.svelte

@@ -30,6 +30,10 @@ import { SvelteURLSearchParams, SvelteDate } from "svelte/reactivity";
     let maxReconnectDelay = 30000;
     let reconnectTimeout;
     let destroyed = false;
+    // Task id whose connection/log stream was initialized; same-id store updates must not re-init.
+    let initializedTaskId: string | null = null;
+    // Task id whose detail refresh already ran; FAILED tasks may legitimately keep result == null.
+    let detailsRefreshedForTaskId: string | null = null;
     let waitingForData = $state(false);
     let dataTimeout;
     let connectionStatus = $state('disconnected'); // 'connecting', 'connected', 'disconnected', 'waiting', 'completed', 'failed', 'awaiting_mapping', 'awaiting_input'
@@ -150,7 +154,7 @@ import { SvelteURLSearchParams, SvelteDate } from "svelte/reactivity";
                 if (st === 'SUCCESS' || st === 'FAILED' || st === 'ERROR' || st === 'COMPLETED') {
                     connectionStatus = st === 'FAILED' || st === 'ERROR' ? 'failed' : 'completed';
                     selectedTask.set({ ...selectedTask.current, ...logEntry.task });
-                    void refreshTaskDetails();
+                    refreshTaskDetailsOnce();
                     try { ws.close(); } catch { /* ignore */ }
                 }
             }
@@ -223,6 +227,8 @@ import { SvelteURLSearchParams, SvelteDate } from "svelte/reactivity";
     // #region Tasks.TaskRunner.RefreshTaskDetails [C:3] [TYPE Function] [SEMANTICS task,status,refresh]
     // @ingroup Tasks
     // @BRIEF Reload task from API so structured plugin result is available for TaskResultPanel.
+    // @POST selectedTask is replaced with the API copy; callers must go through
+    //       refreshTaskDetailsOnce so the resulting store notification cannot re-trigger a refresh.
     async function refreshTaskDetails() {
         const task = selectedTask.current;
         if (!task?.id) return;
@@ -249,6 +255,20 @@ import { SvelteURLSearchParams, SvelteDate } from "svelte/reactivity";
     }
     // #endregion Tasks.TaskRunner.RefreshTaskDetails
 
+    // #region Tasks.TaskRunner.RefreshTaskDetailsOnce [C:2] [TYPE Function] [SEMANTICS task,refresh,guard]
+    // @ingroup Tasks
+    // @BRIEF Run refreshTaskDetails at most once per task id.
+    // @RATIONALE A FAILED task without structured result stays result == null after reload;
+    //            an unguarded refresh -> selectedTask.set -> subscriber -> refresh cycle loops forever.
+    // @POST detailsRefreshedForTaskId === selectedTask.current.id.
+    function refreshTaskDetailsOnce() {
+        const id = selectedTask.current?.id ?? null;
+        if (!id || detailsRefreshedForTaskId === id) return;
+        detailsRefreshedForTaskId = id;
+        void refreshTaskDetails();
+    }
+    // #endregion Tasks.TaskRunner.RefreshTaskDetailsOnce
+
     // #region Tasks.TaskRunner.HandleFilterChangeFunction [TYPE Function]
 // @LAYER UI
     /**
@@ -397,17 +417,39 @@ import { SvelteURLSearchParams, SvelteDate } from "svelte/reactivity";
      * @purpose Initializes WebSocket connection when component mounts.
      * @pre Component must be mounted in DOM.
      * @post WebSocket connection established, subscription to selectedTask active.
+     * @post Connection/log re-initialization happens only when the selected task id changes;
+     *       same-id updates only sync terminal status. Detail refresh runs at most once per task id.
      */
     onMount(() => {
         destroyed = false;
         const unsubscribe = selectedTask.subscribe(task => {
+            if (!task) {
+                initializedTaskId = null;
+                detailsRefreshedForTaskId = null;
+                return;
+            }
             if (task) {
                 const st = String(task.status || '').toUpperCase();
+                const isTerminal = ['SUCCESS', 'FAILED', 'COMPLETED', 'ERROR'].includes(st);
                 const terminalConnectionStatus = ['SUCCESS', 'COMPLETED'].includes(st)
                     ? 'completed'
                     : ['FAILED', 'ERROR'].includes(st)
                       ? 'failed'
                       : null;
+
+                if (task.id && task.id === initializedTaskId) {
+                    // Same task updated (detail refresh or caller polling): no re-init.
+                    if (terminalConnectionStatus) {
+                        connectionStatus = terminalConnectionStatus;
+                        clearTimeout(reconnectTimeout);
+                        if (ws) { try { ws.close(); } catch { /* ignore */ } }
+                        if (task.result == null) refreshTaskDetailsOnce();
+                    }
+                    return;
+                }
+                initializedTaskId = task.id ?? null;
+                detailsRefreshedForTaskId = null;
+
                 log("TaskRunner", "REASON", "Task selected, initializing connection", { taskId: task.id });
                 if (terminalConnectionStatus) connectionStatus = terminalConnectionStatus;
                 if (ws) ws.close();
@@ -446,11 +488,8 @@ import { SvelteURLSearchParams, SvelteDate } from "svelte/reactivity";
                 }
 
                 // Terminal tasks without structured result: reload detail for TaskResultPanel.
-                if (
-                    ['SUCCESS', 'FAILED', 'COMPLETED', 'ERROR'].includes(st) &&
-                    (task.result == null || task.result === undefined)
-                ) {
-                    void refreshTaskDetails();
+                if (isTerminal && task.result == null) {
+                    refreshTaskDetailsOnce();
                 }
 
                 connect();

frontend/src/lib/components/tasks/__tests__/TaskRunner.status.test.ts

@@ -4,11 +4,13 @@
 // @RELATION VERIFIES -> [Tasks.TaskRunner]
 // @TEST_INVARIANT FAILED and ERROR task statuses render failure and never reconnect.
 // @TEST_INVARIANT Ordinary log text containing FAILED does not terminate the task.
+// @TEST_INVARIANT A FAILED task without structured result refreshes details once and never loops.
 
 import { render, screen, waitFor } from '@testing-library/svelte';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { selectedTask, taskLogs } from '$lib/stores/selectedTask.svelte.js';
 import TaskRunner from '../TaskRunner.svelte';
+import { api } from '$lib/api';
 
 vi.mock('$lib/api', () => ({
   getWsUrl: (taskId: string) => `ws://example.test/tasks/${taskId}`,
@@ -141,6 +143,30 @@ describe('TaskRunner structured terminal status', () => {
     expect(FakeWebSocket.instances).toHaveLength(1);
   });
 
+  it('refreshes a FAILED task without result exactly once (no refresh loop)', async () => {
+    vi.mocked(api.getTask).mockClear();
+    vi.mocked(api.getTaskLogs).mockClear();
+    vi.mocked(api.getTask).mockImplementation(async (taskId: string) => ({
+      id: taskId,
+      status: 'FAILED',
+      params: {},
+      result: null,
+    }));
+    selectedTask.set({ id: 'task-no-result', status: 'FAILED', params: {}, result: null });
+
+    render(TaskRunner);
+
+    expect(await screen.findByText('Failed')).toBeTruthy();
+    await new Promise((resolve) => setTimeout(resolve, 200));
+    // Caller (e.g. Mapper polling) re-sets the same task: must not re-initialize.
+    selectedTask.set({ id: 'task-no-result', status: 'FAILED', params: {}, result: null });
+    await new Promise((resolve) => setTimeout(resolve, 200));
+
+    expect(api.getTask).toHaveBeenCalledTimes(1);
+    expect(api.getTaskLogs).toHaveBeenCalledTimes(1);
+    expect(FakeWebSocket.instances).toHaveLength(0);
+  });
+
   it('explains a recovered migration after an intermediate import error', async () => {
     selectedTask.set({
       id: 'task-recovered',