Add missing properties in app_mention event payload

[seratch]

Summary

Refer to slackapi/bolt-python#1107 for the context

Requirements (place an x in each [ ])

• 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.07%. Comparing base (4d97757) to head (6cbaec6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2160   +/-   ##
=======================================
  Coverage   82.07%   82.07%           
=======================================
  Files          18       18           
  Lines        1545     1545           
  Branches      443      443           
=======================================
  Hits         1268     1268           
  Misses        179      179           
  Partials       98       98           

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[zimeg]

Thanks for finding and fixing this 🙏 The commented context and related issue is all super useful to know and all of these fields are appearing as expected in testing 🚢💨