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.

Category (place an x in each of the [ ])

• 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 (place an x in each [ ])

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 90.92%. Comparing base (5aabb9d) to head (3c3f90c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1184   +/-   ##
=======================================
  Coverage   90.92%   90.92%           
=======================================
  Files         221      222    +1     
  Lines        7491     7495    +4     
=======================================
+ Hits         6811     6815    +4     
  Misses        680      680           

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