Guard validate_work_item_type_id with will_save_change check

What does this MR do and why?

Guards HasType#validate_work_item_type_id with will_save_change_to_work_item_type_id? so saves that do not touch work_item_type_id skip validation entirely.

Once type visibility controls are live, a work item can legitimately have a disabled type (the type was disabled after the work item was created). Without this guard, editing the title of such a work item would trigger the validation, fail, and make the work item uneditable.

Also inlines valid_work_item_type_id? (no external callers) and improves the error message.

References

• #591372 (closed)
• Epic: gitlab-org#20061 (closed)

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