Replace data.count with data.size in HasMany field (#2837)

This updates the `HasMany` field to avoid unnecessary `COUNT(*)`
database queries by changing the `more_than_limit?` method to use
`data.size` instead of `data.count(:all)`.

With eager-loaded associations: Administrate preloads child records via
`resources.includes(*includes)` when you configure `collection_includes`
on your dashboard. On an eager-loaded `ActiveRecord::Relation`, calling
`data.size` returns the in-memory `Array#size`, so **no additional SQL**
is issued.

Without eager-loading: If you haven’t configured any `includes`, then
`data` remains an unloaded relation. In that case, `data.size` will fall
back to issuing a `SELECT COUNT(*)`, ensuring the behavior stays correct
but without the benefit of preloading.

In contrast, the original `data.count(:all)` always triggers a `SELECT
COUNT(*)` SQL query, even when the records are already loaded, resulting
in unnecessary database round-trips.

Author: mitchwolfe1

lib/administrate/field/has_many.rb

@@ -72,7 +72,7 @@ def resources(page = 1, order = self.order)
       end
 
       def more_than_limit?
-        paginate? && data.count(:all) > limit
+        paginate? && data.size > limit
       end
 
       def data

spec/support/mock_relation.rb

@@ -17,8 +17,7 @@ def limit(n)
     @data.first(n)
   end
 
-  def count(column = nil)
-    return @data.count if column == :all
-    @data.count(column)
+  def size
+    @data.size
   end
 end