Fix a bug where parsing assistant thread message event fails for beta feature enabled apps

[seratch]

This pull request resolves a bug where parsing assistant thread message event fails for beta feature enabled apps.
See also: slackapi/bolt-python#1184

Summary

Describe the goal of this PR. Mention any related Issue numbers.

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 92.47%. Comparing base (840a26b) to head (c856e8e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2298   +/-   ##
=======================================
  Coverage   92.47%   92.47%           
=======================================
  Files          36       36           
  Lines        7453     7457    +4     
  Branches      646      648    +2     
=======================================
+ Hits         6892     6896    +4     
  Misses        553      553           
  Partials        8        8           

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