Issue #3593113 by mably: Multi-config form registration check requires all config names

Fixes #3593113.

AccountSettingsForm::getEditableConfigNames() lists system.site first, shared with the site information form. isConfigurationRegisteredForDomain() reduced the names to current($names) (the first one), so enabling the override on system.site made the account settings toggler show "remove" even though user.mail and user.settings were never registered.

This checks every name in the set: a multi-config form is registered for a domain only when all of its config names are.

The per-name save/load path is unaffected. doGet() passes single-element arrays and loadMultiple() is immutable (it returns before the registration check), so the only caller passing a genuine multi-name array is the toggler. No routing change is needed.

Also surfaces the affected config names on the toggler as a hover title so the shared-config behavior is discoverable.

Kernel test: a set with only system.site registered is not reported as registered; it is once every name is registered. Fails before this change, passes after.