Handles user not in Slack workspace error

[mvilanova]

Changes Summary

src/dispatch/conversation/flows.py

• Reordered imports for Project and Service models.
• Added a raise e statement in the add_case_participants function to re-raise exceptions.
• Corrected a log message punctuation in add_incident_participants_to_conversation.
• Changed the type of db_session parameter in delete_conversation from SessionLocal to Session.

src/dispatch/participant/flows.py

• Reordered imports for TypeVar, List, and Optional.
• Changed the type of roles parameter in add_participant from Optional[List[ParticipantRoleType]] to Optional[List[str]].

src/dispatch/participant/service.py

• Reordered imports for case_service.

src/dispatch/plugins/dispatch_slack/case/interactive.py

• Added a new message function create_case_user_not_in_slack_workspace_message.
• Improved error handling in the engage function to handle cases where the user email is not found or the user is not in the Slack workspace.
• Reorganized logic to ensure participant addition and engagement text handling.

src/dispatch/plugins/dispatch_slack/case/messages.py

• Added a new function create_case_user_not_in_slack_workspace_message to create a message for users not in the Slack workspace.

src/dispatch/plugins/dispatch_slack/enums.py

• Reordered and added new error codes in SlackAPIErrorCode, including ORG_USER_NOT_IN_TEAM and MISSING_SCOPE.