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'

plugins/chat/spec/requests/chat/api/summaries_controller_spec.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+RSpec.describe Chat::Api::SummariesController do
+  fab!(:current_user) { Fabricate(:user) }
+  fab!(:group) { Fabricate(:group) }
+  let(:plugin) { Plugin::Instance.new }
+
+  before do
+    group.add(current_user)
+
+    strategy = DummyCustomSummarization.new("dummy")
+    plugin.register_summarization_strategy(strategy)
+    SiteSetting.summarization_strategy = strategy.model
+    SiteSetting.custom_summarization_allowed_groups = group.id
+
+    SiteSetting.chat_enabled = true
+    SiteSetting.chat_allowed_groups = group.id
+    sign_in(current_user)
+  end
+
+  describe "#get_summary" do
+    context "when the user is not allowed to join the channel" do
+      fab!(:channel) { Fabricate(:private_category_channel) }
+
+      it "returns a 403" do
+        get "/chat/api/channels/#{channel.id}/summarize", params: { since: 6 }
+
+        expect(response.status).to eq(403)
+      end
+    end
+  end
+end