DEV: Move channel creation for category into service (#21167)

This commit moves the category channel creation out
of the Chat::Api::Channel controller and into a
dedicated CreateCategoryChannel service. A follow up
commit will move the DM channel creation out of
the old DirectMessageChannelCreator service.

Also includes a new on_model_errors helper
for chat service class usage, that collects model
validation errors to present in a nice way.

---------

Co-authored-by: Loïc Guitaut <loic@discourse.org>

plugins/chat/app/models/concerns/chat/chatable.rb

@@ -12,6 +12,10 @@ module Chat
       channel_class.create!(args.merge(chatable: self))
     end
 
+    def create_chat_channel(**args)
+      channel_class.create(args.merge(chatable: self))
+    end
+
     private
 
     def channel_class