FEATURE: Stream topic summaries. (#23065)

When we receive the stream parameter, we'll queue a job that periodically publishes partial updates, and after the summarization finishes, a final one with the completed version, plus metadata.

`summary-box` listens to these updates via MessageBus, and updates state accordingly.

spec/support/dummy_custom_summarization.rb

@@ -22,6 +22,6 @@ class DummyCustomSummarization < Summarization::Base
   end
 
   def summarize(_content)
-    @summarization_result
+    @summarization_result.tap { |result| yield(result[:summary]) if block_given? }
   end
 end