DEV: Move discourse-chat to the core repo. (#18776)

As part of this move, we are also renaming `discourse-chat` to `chat`.

plugins/chat/lib/chat_message_processor.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class Chat::ChatMessageProcessor
+  include ::CookedProcessorMixin
+
+  def initialize(chat_message)
+    @model = chat_message
+    @previous_cooked = (chat_message.cooked || "").dup
+    @with_secure_uploads = false
+    @size_cache = {}
+    @opts = {}
+
+    cooked = ChatMessage.cook(chat_message.message)
+    @doc = Loofah.fragment(cooked)
+  end
+
+  def run!
+    post_process_oneboxes
+    DiscourseEvent.trigger(:chat_message_processed, @doc, @model)
+  end
+
+  def large_images
+    []
+  end
+
+  def broken_images
+    []
+  end
+
+  def downloaded_images
+    {}
+  end
+end