fix(assistant): improve middleware dispatch and inject kwargs in middleware

[WilliamBergamin]

Summary

• Extracts assistant kwargs injection (say, set_status, set_title, set_suggested_prompts, get_thread_context, save_thread_context) from App._init_context / AsyncApp._init_context into
  dedicated listener middleware (AttachingAssistantKwargs / AsyncAttachingAssistantKwargs)
• Generalizes extract_thread_ts to work for all event types (not just assistant events), populating context.thread_ts for any threaded event
• Removes thread_ts from the default Say constructor in BoltContext.say — assistant threading is now handled entirely by the Assistant middleware

Testing

1. Pull this branch and build the project
2. Create a new app with bolt-python-assistant-template
3. Use the new Bolt version in the app
4. There should be no behavior changes 🟢

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

❌ Patch coverage is 94.52055% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.09%. Comparing base (72a90d2) to head (69f8f93).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
slack_bolt/middleware/assistant/assistant.py	77.77%	2 Missing ⚠️
slack_bolt/middleware/assistant/async_assistant.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1456      +/-   ##
==========================================
+ Coverage   90.67%   91.09%   +0.42%     
==========================================
  Files         226      228       +2     
  Lines        7205     7244      +39     
==========================================
+ Hits         6533     6599      +66     
+ Misses        672      645      -27     

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