ci: send a notification of scheduled failing tests

[zimeg]

Summary

This PR sends a notification of scheduled failing tests using the new "regression failures" workflow!

Follows: slackapi/bolt-python#1323

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[zimeg]

Checks are failing with this change - perhaps related to #2506. Planning to investigate this later!

🔗 https://github.com/slackapi/bolt-js/actions/runs/16178549521/job/45669458331?pr=2601

Error: src/Assistant.ts(398,21): error TS18048: 'payload.assistant_thread' is possibly 'undefined'.
Error: src/Assistant.ts(401,5): error TS2322: Type 'unknown' is not assignable to type 'string'.
Error: src/Assistant.ts(402,5): error TS2322: Type 'unknown' is not assignable to type 'string'.
Error: src/Assistant.ts(403,5): error TS2322: Type 'unknown' is not assignable to type 'AssistantThreadContext'.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.37%. Comparing base (1ab9b1a) to head (6687aaa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2601   +/-   ##
=======================================
  Coverage   93.37%   93.37%           
=======================================
  Files          37       37           
  Lines        7581     7581           
  Branches      667      667           
=======================================
  Hits         7079     7079           
  Misses        497      497           
  Partials        5        5           

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[zimeg]

🗣️ If this seems alright the SLACK_REGRESSION_FAILURES_WEBHOOK_URL secret will be saved before merge!

[WilliamBergamin]

This seems good 💯 I originally hesitated on this since we may get notified when tests are being flaky, but as you brilliantly pointed out it will force us improve the stability of our tests (as they should be)

[zimeg]

@WilliamBergamin Thanks so much 🙏 ✨

I'm optimistic that insight into tests might point out patterns that we might be missing elsewhere, but I also hope for tests to not fail without good reason! So I will merge this.