Bug description

When fetching Slack channels for Alerts & Reports, the code uses a hardcoded max_retry_count=2 in the RateLimitErrorRetryHandler. This causes failures when Slack API returns HTTP 429 (rate limit):

POST https://slack.com/api/conversations.list - HTTP Error 429: Too Many Requests

Reproduction steps:

1. Configure Slack integration with SLACK_API_TOKEN
2. Enable ALERT_REPORTS feature flag
3. Create an Alert/Report with Slack notification
4. Try to select a channel from the dropdown
5. Error occurs after 2 retry attempts

Expected behavior:
Retry count should be configurable to handle rate limits in different workspace sizes.

Root cause:
superset/utils/slack.py line 54:

rate_limit_handler = RateLimitErrorRetryHandler(max_retry_count=2)  # Hardcoded

Screenshots/recordings

N/A

Superset version

master / latest-dev

Python version

3.11

Node version

16

Browser

Chrome

Additional context

• More noticeable in large workspaces (10k+ channels) due to higher number of paginated requests
• Feature flag enabled: ALERT_REPORTS
• Fix available in PR #35622

Checklist

• I have searched Superset docs and Slack and didn't find a solution to my problem.
• I have searched the GitHub issue tracker and didn't find a similar bug report.
• I have checked Superset's logs for errors and if I found a relevant Python stacktrace, I included it here as text in the "additional context" section.