Improve metadata resolution timing in assistant app's say method

[seratch]

This pull request improves the Assistant middleware's internals. Like the asyncio version already does, the get_thread_context method call should be delayed until it's really necessary: https://github.com/slackapi/bolt-python/blob/v1.21.1/slack_bolt/context/assistant/async_assistant_utilities.py#L69-L80

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 90.93%. Comparing base (18a9135) to head (96201d6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1186      +/-   ##
==========================================
+ Coverage   90.92%   90.93%   +0.01%     
==========================================
  Files         222      222              
  Lines        7495     7505      +10     
==========================================
+ Hits         6815     6825      +10     
  Misses        680      680              

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[seratch]

Thanks for the swift review!