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/spec/support/chat_service_matcher.rb

@@ -84,6 +84,24 @@ module Chat
       def description
         "fail to find a model named '#{name}'"
       end
+
+      def step_failed?
+        super && result[name].blank?
+      end
+    end
+
+    class FailWithInvalidModel < FailStep
+      def type
+        "model"
+      end
+
+      def description
+        "fail to have a valid model named '#{name}'"
+      end
+
+      def step_failed?
+        super && result[step].invalid
+      end
     end
 
     def fail_a_policy(name)
@@ -98,6 +116,10 @@ module Chat
       FailToFindModel.new(name)
     end
 
+    def fail_with_an_invalid_model(name = "model")
+      FailWithInvalidModel.new(name)
+    end
+
     def inspect_steps(result)
       inspector = Chat::StepsInspector.new(result)
       puts "Steps:"