fix(hitl): emit execution:cancelled event to canvas when cancelling paused execution

Paused HITL executions have no active SSE stream, so the canvas never
received the cancellation event. Now writes execution:cancelled to the
event buffer and updates the stream meta so the canvas reconnect path
picks it up and shows 'Execution Cancelled'.

Author: waleedlatif1

apps/sim/app/api/workflows/[id]/executions/[executionId]/cancel/route.test.ts

@@ -11,12 +11,18 @@ const {
   mockMarkExecutionCancelled,
   mockAbortManualExecution,
   mockCancelPausedExecution,
+  mockSetExecutionMeta,
+  mockWriteEvent,
+  mockCloseWriter,
 } = vi.hoisted(() => ({
   mockCheckHybridAuth: vi.fn(),
   mockAuthorizeWorkflowByWorkspacePermission: vi.fn(),
   mockMarkExecutionCancelled: vi.fn(),
   mockAbortManualExecution: vi.fn(),
   mockCancelPausedExecution: vi.fn(),
+  mockSetExecutionMeta: vi.fn(),
+  mockWriteEvent: vi.fn(),
+  mockCloseWriter: vi.fn(),
 }))
 
 vi.mock('@/lib/auth/hybrid', () => ({
@@ -46,6 +52,14 @@ vi.mock('@/lib/posthog/server', () => ({
   captureServerEvent: vi.fn(),
 }))
 
+vi.mock('@/lib/execution/event-buffer', () => ({
+  setExecutionMeta: (...args: unknown[]) => mockSetExecutionMeta(...args),
+  createExecutionEventWriter: () => ({
+    write: (...args: unknown[]) => mockWriteEvent(...args),
+    close: () => mockCloseWriter(),
+  }),
+}))
+
 import { POST } from './route'
 
 const makeRequest = () =>
@@ -62,6 +76,9 @@ describe('POST /api/workflows/[id]/executions/[executionId]/cancel', () => {
     mockAuthorizeWorkflowByWorkspacePermission.mockResolvedValue({ allowed: true })
     mockAbortManualExecution.mockReturnValue(false)
     mockCancelPausedExecution.mockResolvedValue(false)
+    mockSetExecutionMeta.mockResolvedValue(undefined)
+    mockWriteEvent.mockResolvedValue({ eventId: 1 })
+    mockCloseWriter.mockResolvedValue(undefined)
   })
 
   it('returns success when cancellation was durably recorded', async () => {

apps/sim/app/api/workflows/[id]/executions/[executionId]/cancel/route.ts

@@ -2,6 +2,7 @@ import { createLogger } from '@sim/logger'
 import { type NextRequest, NextResponse } from 'next/server'
 import { checkHybridAuth } from '@/lib/auth/hybrid'
 import { markExecutionCancelled } from '@/lib/execution/cancellation'
+import { createExecutionEventWriter, setExecutionMeta } from '@/lib/execution/event-buffer'
 import { abortManualExecution } from '@/lib/execution/manual-cancellation'
 import { captureServerEvent } from '@/lib/posthog/server'
 import { PauseResumeManager } from '@/lib/workflows/executor/human-in-the-loop-manager'
@@ -58,6 +59,17 @@ export async function POST(
       logger.info('Execution cancelled via local in-process fallback', { executionId })
     } else if (pausedCancelled) {
       logger.info('Paused execution cancelled directly in database', { executionId })
+      void setExecutionMeta(executionId, { status: 'cancelled', workflowId })
+      const writer = createExecutionEventWriter(executionId)
+      void writer
+        .write({
+          type: 'execution:cancelled',
+          timestamp: new Date().toISOString(),
+          executionId,
+          workflowId,
+          data: { duration: 0 },
+        })
+        .then(() => writer.close())
     } else {
       logger.warn('Execution cancellation was not durably recorded', {
         executionId,