* feat(nested folders): add CountDashboardsInFolder
This commit adds a new method to the Dashboard service and stores: CountDashboardsInFolder. The command struct takes a folderUID, but the store implementation still depends on the parent folder ID. This is temporary; eventually we will replace all references to FolderIDs (associated with Dashboards) with folder UIDs.
There are some unfortunate additional test changes that were necessary after generating the service & store mocks; it looks like that hasn't been generated since the last change(s).
* more test updates
* don't forget the service test
* that didn't end up used, so bye for now
* agree to disagree with the linter
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