Integrate work items into chat notifications as issue events

What does this MR do and why?

Add support for work_item and confidential_work_item events in chat notifications by treating them as issue events. This provides consistent notification behavior across all work item types.

This fixes Service Desk tickets not triggering Slack notifications after the recent Service Desk migration from legacy issues to work items of type ticket.

Changes

• Add work_item and confidential_work_item to SUPPORTED_EVENTS
• Include work_item events in SUPPORTED_EVENTS_FOR_LABEL_FILTER
• Map work_item events to issue events in map_work_item_event
• Route work items through IssueMessage for consistent formatting
• Apply work item event mapping in should_execute, notify_label, channels_for_event, and execute methods
• Override configurable_events to exclude work item events from UI since they use issue event settings
• Skip incident work items in work item flow to handle separately, to avoid double notifications

This ensures work items integrate seamlessly into existing chat notification workflows without requiring separate channels or message formats, and restores Service Desk notification functionality.

Notes

• We'll change the wording in chat integrations to use 'work item' instead of 'issues' after the saved views feature flag has been enabled via #585877.

References

• #585706 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Setup

1. Create a new Slack workspace, since non-admin users aren't allowed to install apps in the GitLab workspace.
2. Create a GitLab for Slack app.
3. Configure the app settings in GitLab.
4. Install the GitLab for Slack app.

Before the fix

• When creating an issue, incident or work items like task or Service Desk ticket, notifications were only triggered for issues and incidents.
• Note: To create a Service Desk ticket, follow the instructions here.

After the fix

• When creating an issue, incident, or other work items (such as tasks or Service Desk tickets), notifications are now triggered for all item types.

MR acceptance checklist

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