Fix `todoCreate` mutation for group level issues/work_items

What does this MR do and why?

Now that an issue can belong directly to a group, readable_by? needs to be able to handle this.

No change to the authorization logic, so policy remains in sync with the method, including the abilities a user might have for a group level issue.

We had no unit specs for this method, so I added as much as I could without creating several projects. Perhaps we can add what's missing as a follow up since the new coverage is already an improvement and definitively covers group level issues which is the addition.

We also needed to update the TodoService to handle issues/work_items that don't belong to a project but directly to a group

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:namespace_level_work_items)
2. Create a group level work item with the following query at http://127.0.0.1:3000/-/graphql-explorer so you can fetch a work item in the next query

   mutation {
      workItemCreate(input: {namespacePath: "flightjs", title: "test group level work item", workItemTypeId: "gid://gitlab/WorkItems::Type/1"}) {
        errors
        workItem {
          id
          iid
        }
      }
    }

3. Fetch a wok item by IID and request the WorkItemWidgetLinkedItems field

    mutation {
      todoCreate(input: {targetId: "<id_returned_in_previous_step>"}) {
        errors
        todo {
          action
          author { username }
          target {
            webUrl
          }
          targetType
        }
      }
    }

   Before this change, you will get and undefined method for nil error

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #426466 (closed)