DEV: Migrate Chat::MessageCreator to a service (#22390)

Currently, the logic for creating a new chat message is scattered
between a controller and an “old” service.

This patch address this issue by creating a new service (using the “new”
sevice object system) encapsulating all the necessary logic.
(authorization, publishing events, etc.)

plugins/chat/app/policies/chat/channel/message_creation_policy.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+class Chat::Channel::MessageCreationPolicy < PolicyBase
+  class DirectMessageStrategy
+    class << self
+      def call(guardian, channel)
+        guardian.can_create_channel_message?(channel) || guardian.can_create_direct_message?
+      end
+
+      def reason(*)
+        I18n.t("chat.errors.user_cannot_send_direct_messages")
+      end
+    end
+  end
+
+  class CategoryStrategy
+    class << self
+      def call(guardian, channel)
+        guardian.can_create_channel_message?(channel)
+      end
+
+      def reason(_, channel)
+        I18n.t("chat.errors.channel_new_message_disallowed.#{channel.status}")
+      end
+    end
+  end
+
+  attr_reader :strategy
+
+  delegate :channel, to: :context
+
+  def initialize(*)
+    super
+    @strategy = CategoryStrategy
+    @strategy = DirectMessageStrategy if channel.direct_message_channel?
+  end
+
+  def call
+    strategy.call(guardian, channel)
+  end
+
+  def reason
+    strategy.reason(guardian, channel)
+  end
+end