* pgrade Slack integration to use AsyncWebClient and support files_upload_v2
- Replaced deprecated WebClient with AsyncWebClient throughout the integration.
- Removed the unsupported `run_async` parameter.
- Added a helper function to resolve channel names to channel IDs.
- Updated `_async_send_local_file_message` and `_async_send_remote_file_message` to handle Slack's new API requirements, including per-channel uploads.
- Updated dependency from slackclient==2.5.0 to slack-sdk>=3.0.0.
- Improved error handling and logging for channel resolution and file uploads.
* Fix test to use AsyncWebClient for Slack authentication flow
* Fix Slack authentication URL by removing the www subdomain
* Refactor Slack file upload functionality and add utility for file uploads
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