chore: improve testing around assistant utilities

[WilliamBergamin]

Summary

These changes aim to improve the coverage of tests around the assistant utilities

Testing

CI should be sufficient

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.89%. Comparing base (898e0b8) to head (5a75303).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## main #1461 +/- ##
==========================================
+ Coverage 90.67% 90.89% +0.22% 
==========================================
Files 226 226 Lines 7205 7205 ==========================================
+ Hits 6533 6549 +16 + Misses 672 656 -16 

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[zimeg]

@WilliamBergamin LGTM! These are nice tests to build confidence in current implementation 🧪 ✨