fix: prevent post data refetch with finishResolution instead of middleware filtering

Call finishResolution('getEntityRecord', ...) after receiveEntityRecords in
useEditorSetup so Gutenberg's data layer knows the entity is already resolved.
This eliminates the need for filterEndpointsMiddleware, which previously blocked
the refetch by intercepting GET requests to the post endpoint.

Closes #434

Author: jkmassel