Slack channel not listed in alert dropdown, despite bot being in channel and API access confirmed

### Describe the bug

I'm using Metabase Cloud (v1.54.9.2, Starter Plan). I set up a custom Slack App with the following scopes:

chat:write

channels:read

groups:read

I:

Installed the app and copied the Bot User OAuth Token

Invited the bot (@Metabase Alerta) to the target channel

Verified via the Slack API (conversations.list) that the channel appears

Tested chat.postMessage and confirmed it sends messages to the channel correctly

However, in Metabase, when I go to create an alert and choose Slack, the dropdown never shows any channels, and pasting the Channel ID directly also fails silently.

There’s no error in UI or logs. The Slack token is connected and saved.

➜ Expected: Channels visible in dropdown (or at least manual Channel ID accepted)

➜ Actual: No channel appears, even though the bot is confirmed to be in the channel and working.

Please advise. Slack App ID: A08TKA7HQ59. Channel ID: C08T8N9B0F2

### To Reproduce

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

### Expected behavior

_No response_

### Logs

_No response_

### Information about your Metabase installation

```JSON
{
"browser-info": {
"language": "en-US",
"platform": "Win32",
"userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36",
"vendor": "Google Inc."
},
"metabase-info": {
"databases": [
"postgres",
"redshift",
"h2"
],
"run-mode": "prod",
"plan-alias": "starter",
"version": {
"date": "2025-05-21",
"tag": "v1.54.9.2",
"hash": "9b67f93"
},
"settings": {
"report-timezone": null
},
"hosting-env": "unknown",
"application-database": "postgres"
}
}
```

### Severity

A feature isn’t working correctly

### Additional context

_No response_

No source files or tests are named. Start by reproducing the alert Slack channel selection with the reported Metabase Cloud version and compare it with the Slack conversations.list and chat.postMessage checks described; done means an invited channel appears in the dropdown or a valid Channel ID is accepted, with an observable error when it is not.

Written by the indexing model from the issue text.