Prevent invalid type options from displaying in the Add Existing dropdown for Child widget when using URL

Summary

When attempting to add a Child item, a user can paste the URL of an invalid child type (ex: a Task when adding a child to an Epic), which shows and allows the resulting child as an option. However, when submitting the form the user is presented with an error that the item cannot be added as "it's not allowed to add this type of parent item" – which is quite confusing.

Proposal

We shouldn't show options for items that are invalid child types within the token selector dropdown, even when the direct URL is used. Similarly, we don't show non-valid type options in the default option list (prior to searching) or when searching by keyword.

Example of trying to use keyword to find that option

search title	search id