Fix CustomFields callback crashing during work item type change

What does this MR do?

CustomFields callback crashes when converting w... (#599287 - closed)

`/convert_to_ticket` and `/promote_to_incident`... (#594250 - closed)

Fixes a crash in WorkItems::Callbacks::CustomFields#after_save that occurs when a work item's type is changed to a type that does not include the custom_fields widget (e.g. converting an issue to a Service Desk ticket via the /convert_to_ticket quick action, or using /type to change to a type without custom fields support).

Root cause: When WorkItems::UpdateService processes a type change, WidgetableService#initialize_callbacks! iterates all widgets on the current work item. For any widget not supported by the new type, it stamps excluded_in_new_type: true onto the callback params. This activates the callback but leaves params as { excluded_in_new_type: true } — a Hash, not the Array of field hashes that CustomFields#after_save expects. The call to params.pluck(:custom_field_id) then raises:

Internal server error: no implicit conversion of Symbol into Integer

Fix: Add the excluded_in_new_type? early return that every other callback (weight, iteration, health_status, color, status, etc.) already has. One line added, consistent with the established pattern.

Steps to validate locally

Scenario 1 — /convert_to_ticket quick action (main trigger):

1. Set up your instance to activate incoming email and Service Desk (use gitlab.yml portion of the snippet)
2. Open an issue
3. Add a comment with /convert_to_ticket user@example.com
4. Save

Before this fix: GraphQL returns Internal server error: no implicit conversion of Symbol into Integer and the conversion fails. After this fix: the issue is successfully converted to a Service Desk ticket.

Scenario 2 — /type quick action:

1. Open an issue
2. Use /type Task (or any other type that also has custom_fields in its widget list — this won't reproduce the crash; use a type without it)
3. To reproduce, target a type that does not include custom_fields

Note: most system-defined types do include custom_fields, so ticket (via /convert_to_ticket) is currently the clearest repro path.

Closes #599287 (closed)

Closes #594250 (closed)