FIX: Trashing message should reset last read (#20912)

When a chat message is trashed and the message is used
for someone's UserChatChannelMembership#last_read_message_id,
the user would end up with some read state issues until
someone posted a new message in the channel, since we didn't
clear it like we did on bulk message delete.

This commit fixes the issue, and also takes the opportunity
to start a MessagesController in the API namespace, and move
the trash message functionality into the new service format.

plugins/chat/app/services/chat/message_destroyer.rb

@@ -12,19 +12,6 @@ module Chat
         end
     end
 
-    def trash_message(message, actor)
-      Chat::Message.transaction do
-        message.trash!(actor)
-        Chat::Mention.where(chat_message: message).destroy_all
-        DiscourseEvent.trigger(:chat_message_trashed, message, message.chat_channel, actor)
-
-        # FIXME: We should do something to prevent the blue/green bubble
-        # of other channel members from getting out of sync when a message
-        # gets deleted.
-        Chat::Publisher.publish_delete!(message.chat_channel, message)
-      end
-    end
-
     private
 
     def reset_last_read(message_ids)

plugins/chat/app/services/chat/trash_message.rb

@@ -0,0 +1,70 @@
+# frozen_string_literal: true
+
+module Chat
+  # Service responsible for trashing a chat message
+  # for a channel and ensuring that the client and read state is
+  # updated.
+  #
+  # @example
+  #  Chat::TrashMessage.call(message_id: 2, channel_id: 1, guardian: guardian)
+  #
+  class TrashMessage
+    include Service::Base
+
+    # @!method call(message_id:, channel_id:, guardian:)
+    #   @param [Integer] message_id
+    #   @param [Integer] channel_id
+    #   @param [Guardian] guardian
+    #   @return [Service::Base::Context]
+
+    contract
+    model :message
+    policy :invalid_access
+    transaction do
+      step :trash_message
+      step :destroy_mentions
+      step :update_tracking_state
+    end
+    step :publish_events
+
+    # @!visibility private
+    class Contract
+      attribute :message_id, :integer
+      attribute :channel_id, :integer
+      validates :message_id, presence: true
+      validates :channel_id, presence: true
+    end
+
+    private
+
+    def fetch_message(contract:, **)
+      Chat::Message.includes(chat_channel: :chatable).find_by(
+        id: contract.message_id,
+        chat_channel_id: contract.channel_id,
+      )
+    end
+
+    def invalid_access(guardian:, message:, **)
+      guardian.can_delete_chat?(message, message.chat_channel.chatable)
+    end
+
+    def trash_message(message:, **)
+      message.trash!
+    end
+
+    def destroy_mentions(message:, **)
+      Chat::Mention.where(chat_message: message).destroy_all
+    end
+
+    def update_tracking_state(message:, **)
+      Chat::UserChatChannelMembership.where(last_read_message_id: message.id).update_all(
+        last_read_message_id: nil,
+      )
+    end
+
+    def publish_events(guardian:, message:, **)
+      DiscourseEvent.trigger(:chat_message_trashed, message, message.chat_channel, guardian.user)
+      Chat::Publisher.publish_delete!(message.chat_channel, message)
+    end
+  end
+end