DEV: Cache summarization strategy results. (#22230)

Updates the interface for implementing summarization strategies and adds a cache layer to summarize topics once.

The cache stores the final summary and each chunk used to build it, which will be useful when we have to extend or rebuild it.

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

@@ -27,7 +27,7 @@ class Chat::Api::SummariesController < Chat::ApiController
           .map { "#{_1}: #{_2}" }
           .join("\n")
 
-      render json: { summary: strategy.summarize(content) }
+      render json: { summary: strategy.summarize(content).dig(:summary) }
     end
   end
 end