Bug Report: “execution_success” fires before outputs are persisted

### Custom Node Testing

- [x] I have tried disabling custom nodes and the issue persists (see [how to disable custom nodes](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled) if you need help)

### Expected Behavior

The completion notification that clients rely on (execution_success / job_completed) should be emitted after outputs are fully persisted and visible via history/status APIs, so fetching outputs immediately after the completion event returns the final artifacts.

### Actual Behavior

execution_success is emitted in execute_async before history_result is written to history in task_done. Clients that fetch job status/outputs as soon as they receive the completion event can observe output_files empty and persist outputs: [], even though the workflow produced artifacts. This is timing-dependent and intermittent.

### Steps to Reproduce

Steps to Reproduce (race)
Run a workflow that produces outputs (e.g., unified SRT TTS) through ComfyUI.
Listen for the websocket execution_success (or downstream job_completed) event.
Immediately upon receiving the event, call the job status/output endpoint (e.g., /api/jobs/{prompt_id} or mediagen’s /mediagen/api/v1/jobs/{jobid}).
Observe that output_files can be empty right after the completion event; later calls (or manual refresh) show the outputs once history finishes persisting.

Impact
Queue/UX or integrations that fetch once on completion can store outputs: [] and never backfill, so completed jobs appear with missing audio/SRT/media.

### Debug Logs

```powershell
Exact line locations in the ComfyUI repo:
ComfyUI/execution.py
Line 731: self.add_message("execution_success", { "prompt_id": prompt_id }, broadcast=False)
Lines 733–741: Build history_result with ui_outputs/meta_outputs (outputs not yet persisted)
ComfyUI/execution.py
Lines 1135–1155: PromptQueue.task_done(...) persists history_result into self.history and triggers queue_updated
ComfyUI/main.py
Lines 200–236: Main loop runs e.execute(...) then calls q.task_done(...) (persistence happens only after execute_async returns)
```

### Other

Suggested Fix (one of)
Move/emit the completion notification after task_done persists outputs (or add a new “execution_persisted” event and have clients wait for it).
Alternatively, include finalized outputs in the completion payload so clients don’t have to race the history API.

Start with ComfyUI/execution.py around the execution_success emission and history_result construction, then trace PromptQueue.task_done in execution.py and the execute/task_done flow in main.py. Reproduce the race by listening for execution_success and immediately querying the job status/output endpoint. Done means completion consumers can fetch persisted outputs without observing an empty output_files result.

Written by the indexing model from the issue text.