FEATURE: Enforce mention limits for chat messages (#19034)

* FEATURE: Enforce mention limits for chat messages

The first part of these changes adds a new setting called `max_mentions_per_chat_message`, which skips notifications when the message contains too many mentions. It also respects the `max_users_notified_per_group_mention` setting
and skips notifications if expanding a group mention would exceed it.

We also include a new component to display JIT warning for these limits to the user while composing a message.

* Simplify ignoring/muting filter in chat_notifier

* Post-send warnings for unsent warnings

* Improve pluralization

* Address review feedback

* Fix test

* Address second feedback round

* Third round of feedback

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

plugins/chat/app/controllers/api/hints_controller.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+class Chat::Api::HintsController < ApplicationController
+  before_action :ensure_logged_in
+
+  def check_group_mentions
+    RateLimiter.new(current_user, "group_mention_hints", 5, 10.seconds).performed!
+    group_names = params[:mentions]
+
+    raise Discourse::InvalidParameters.new(:mentions) if group_names.blank?
+
+    visible_groups = Group
+      .where("LOWER(name) IN (?)", group_names)
+      .visible_groups(current_user)
+      .pluck(:name)
+
+    mentionable_groups = filter_mentionable_groups(visible_groups)
+
+    result = {
+      unreachable: visible_groups - mentionable_groups.map(&:name),
+      over_members_limit: mentionable_groups.select { |g| g.user_count > SiteSetting.max_users_notified_per_group_mention }.map(&:name),
+    }
+
+    result[:invalid] = (group_names - result[:unreachable]) - result[:over_members_limit]
+
+    render json: result
+  end
+
+  private
+
+  def filter_mentionable_groups(group_names)
+    return [] if group_names.empty?
+
+    Group
+      .select(:name, :user_count)
+      .where(name: group_names)
+      .mentionable(current_user, include_public: false)
+  end
+end

plugins/chat/app/services/chat_publisher.rb

@@ -153,23 +153,19 @@ module ChatPublisher
     user_id,
     chat_message,
     cannot_chat_users,
-    without_membership
+    without_membership,
+    too_many_members,
+    mentions_disabled
   )
     MessageBus.publish(
       "/chat/#{chat_message.chat_channel_id}",
       {
         type: :mention_warning,
         chat_message_id: chat_message.id,
-        cannot_see:
-          ActiveModel::ArraySerializer.new(
-            cannot_chat_users,
-            each_serializer: BasicUserSerializer,
-          ).as_json,
-        without_membership:
-          ActiveModel::ArraySerializer.new(
-            without_membership,
-            each_serializer: BasicUserSerializer,
-          ).as_json,
+        cannot_see: cannot_chat_users.map { |u| { username: u.username, id: u.id } }.as_json,
+        without_membership: without_membership.map { |u| { username: u.username, id: u.id } }.as_json,
+        groups_with_too_many_members: too_many_members.map(&:name).as_json,
+        group_mentions_disabled: mentions_disabled.map(&:name).as_json
       },
       user_ids: [user_id],
     )