fix(assistant): accept plain functions as listener middleware - #1591

Summary

Assistant and AsyncAssistant listener decorators (thread_started, user_message, and so on) are typed to accept middleware as a list of plain functions or Middleware objects, the same as App.event(). But build_listener passed the list straight to CustomListener without wrapping functions, so a plain function middleware raised:

AttributeError: 'function' object has no attribute 'process'

on every matching event, which became a 500.

A second problem in the same code: build_listener inserted AttachingConversationKwargs into the caller's list in place. A list shared between two decorators ended up with that middleware added twice.

Changes in slack_bolt/middleware/assistant/:

• Plain functions are wrapped in CustomMiddleware (sync) or AsyncCustomMiddleware (async), matching what App._register_listener does. Anything else raises the same BoltError that App raises.
• A new middleware list is built for each listener, so the caller's list is left untouched.
• The build_listener type hint now matches the decorator type hints.

Testing

Added test_assistant_with_function_listener_middleware to both tests/scenario_tests/test_events_assistant.py and tests/scenario_tests_async/test_events_assistant.py. Each registers a function middleware on two listeners, checks the middleware and listener both run, and checks the caller's list is unchanged. Both fail on main and pass with this change. Ran ./scripts/format.sh, ./scripts/lint.sh, ./scripts/run_mypy.sh, and the assistant test 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
• Others

Requirements

• 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.

Fixes #1597