[slack] slack-stream: failed to stop stream: Error: An API error occurred: user_not_found

### Steps to reproduce

Enable Slack native streaming:
channels.slack.streaming.mode = "partial"
channels.slack.streaming.nativeTransport = true
Use OpenClaw in a Slack shared channel / thread involving a Slack Connect user
Ask the bot to reply in-thread
Check delivery behavior and gateway logs

### Expected behavior

The final reply should be delivered reliably in the Slack thread.

If native streaming is not supported for that user/thread, OpenClaw should fall back cleanly to normal message delivery without losing the response.

### Actual behavior

The reply is generated internally, but Slack streaming fails during stop/finalize with:

[slack] slack-stream: failed to stop stream: Error: An API error occurred: user_not_found
I also observed this related log line during debugging:

[slack] slack-stream: streaming API call failed: Error: An API error occurred: user_not_found, falling back
So fallback may exist for streaming start/call failure, but finalize/stop failure still appears to cause unreliable delivery.

### OpenClaw version

2026.4.14

### Operating system

macOS 26.4.1

### Install method

npm

### Model

chatgpt 5.4

### Provider / routing chain

Slack Connect shared channel/thread → Slack Socket Mode → local OpenClaw Gateway (macOS) → agent/session routing (agentname) → OpenAI Codex gpt-5.4 for response generation → OpenClaw Slack transport → Slack Web API / native Slack text streaming. No additional proxy or external model router in the Slack delivery path.

### Additional provider/model setup details

Slack mode: socket mode
Slack config:
channels.slack.streaming.mode = "partial"
channels.slack.streaming.nativeTransport = true
channels.slack.replyToMode = "first"



### Logs, screenshots, and evidence

```shell