Add explicit Assistant listener registration mode

[Cr1stal]

Summary

Addresses #1346 by adding an explicit Assistant registration mode to App.assistant() and AsyncApp.assistant().

By default, app.assistant(assistant) continues to behave as it does today: it registers Assistant as middleware. This preserves the existing Assistant middleware dispatch path and keeps app.use(assistant) / app.middleware(assistant) behavior unchanged.

Apps can now opt into listener-based registration:

app.assistant(assistant, mode="listeners")

In this mode, Assistant handlers are registered as normal App listeners. This lets assistant-specific handlers participate in the App listener pipeline and inherit app-level middleware such as authorization, auditing, rate limiting, or request enrichment.

The PR also adds an internal listener registry so Assistant listeners can be evaluated before broad catch-all listeners without exposing a general listener priority API. Registration order is preserved within Assistant listeners and ordinary App listeners, and sync/async behavior is kept mirrored.

Testing

• ./scripts/format.sh --no-install
• ./scripts/lint.sh --no-install
• ./scripts/run_tests.sh tests/slack_bolt/app/test_app_assistant_middleware.py (14 passed)
• ./scripts/run_mypy.sh --no-install (Success: no issues found in 235 source files)
• ./scripts/install_all_and_run_tests.sh (924 passed, 81 warnings; mypy: Success: no issues found in 235 source files)

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.