FEATURE: Custom content summarization strategies. (#21813)

* FEATURE: Content custom summarization strategies.

This PR establishes a pattern for plugins to register alternative ways of summarizing content by extending a class that defines an interface.

Core controls which strategy we'll use and who has access to it through the `summarization_strategy` and `custom_summarization_allowed_groups`. It also defines the UI for summarizing topics.

Other plugins can access this summarization mechanism and implement their features, removing cross-plugin customizations, as it currently happens between chat and the discourse-ai plugin.

* Group membership validation and rate limiting

* Work with objects instead of classes

* Port summarization feature from discourse-ai to chat

* Rename available summaries to 'Top Replies' and 'Summary'

app/controllers/topics_controller.rb

@@ -30,6 +30,7 @@ class TopicsController < ApplicationController
                    publish
                    reset_bump_date
                    set_slow_mode
+                   summary
                  ]
 
   before_action :consider_user_for_promotion, only: :show
@@ -1167,6 +1168,30 @@ class TopicsController < ApplicationController
     head :ok
   end
 
+  def summary
+    topic = Topic.find(params[:topic_id])
+    guardian.ensure_can_see!(topic)
+    strategy = Summarization::Base.selected_strategy
+    raise Discourse::NotFound.new unless strategy
+
+    raise Discourse::InvalidAccess unless strategy.can_request_summaries?(current_user)
+
+    RateLimiter.new(current_user, "summary", 6, 5.minutes).performed!
+
+    hijack do
+      summary_opts = {
+        filter: "summary",
+        exclude_deleted_users: true,
+        exclude_hidden: true,
+        show_deleted: false,
+      }
+
+      content = TopicView.new(topic, current_user, summary_opts).posts.pluck(:raw).join("\n")
+
+      render json: { summary: strategy.summarize(content) }
+    end
+  end
+
   private
 
   def topic_params