FIX: ActiveModel::Types :array load error in production (#22377)

Followup to 3f1024de7698fa357d5b517b37fcb96647fcbe6e

The ActiveModel::Types.register(:array) call for chat was
called too late in the Zeitwerk load order in production,
causing this error:

> `lookup': Unknown type :array (ArgumentError)
> raise ArgumentError, "Unknown type #{symbol.inspect}"
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We need to load the type and register it manually before the rest
of the chat files are loaded via the engine and Zeitwerk.

plugins/chat/lib/chat/types/array.rb

@@ -18,3 +18,5 @@ module Chat
     end
   end
 end
+
+ActiveSupport.on_load(:active_record) { ActiveModel::Type.register(:array, Chat::Types::Array) }