The old names of the class and method were misleading. We don't
actually need any relation. And we don't even need a model instance
as we were returning previously -- the only use of that method was
in TriggerSyncingEvents which would immediately use ::class on the
returned value. Therefore, all we are asking for in this interface
is just the central resource class.
Also make all resource syncing-related listener closures static.
Also correct return type for getGlobalIdentifierKey to string|int.
(We intentionally do not support returning null like many other
"get x key" methods would since such a case might break resource
syncing logic. This is also why we use inline getAttribute() in the
creating listener instead of calling the method.)
Also move pivot record deletion to that listener and improve tests
The 'tenant pivot records are deleted along with the tenants to which
they belong to' test is failing in this commit -- the listener
for deleting mappings when a *tenant* is deleted is only implemented
in the next commit. The only change done here is to re-add FKs
(necessary for passing *in this commit* in that specific dataset
variant) that were removed from the default test migration as we now
have the DeleteResourceMapping listener that's enabled by default.
