feat(slack): remove signal text from manual mfa engage and add user to channel

[whitdog47]

This pull request introduces several key updates to the manual MFA engagement process and enhances user management within Slack channels. The changes aim to streamline the user engagement workflow and improve the clarity of engagement messages.

Key Changes

1. Removal of Signal Text from Manual MFA Engagement:

   • The signal text has been removed from the manual MFA engagement process to simplify the message structure and focus on essential information.

2. Addition of User to Slack Channel:

   • Implemented functionality to add users to the relevant Slack channel during the engagement process, ensuring they are part of the conversation and can receive necessary updates.

3. Code Refactoring:

   • Refactored the handle_engage_user_command and engage functions to improve readability and maintainability.
   • Consolidated duplicate code blocks and optimized the logic for selecting and engaging users.

4. Enhanced Error Handling:

   • Improved error logging for scenarios where participants or cases are not found, aiding in debugging and system reliability.

5. Updated Engagement Message Creation:

   • Modified the create_manual_engagement_message function to ensure engagement messages are clear and contextually relevant.
   • Added checks to handle cases where no engagement text is provided, defaulting to a standard message.

Files Affected

• src/dispatch/plugins/dispatch_slack/case/interactive.py: Major updates to user engagement logic and error handling.
• src/dispatch/plugins/dispatch_slack/case/messages.py: Adjustments to message creation functions for better clarity and structure.