improvements(ai): Improve AI streaming UI/UX interactions + better separation of AI provider responsibilities (#2039)

* Start refactor

* Interface updates

* Rework Assistant, Provider, and tests for better domain boundaries

* Consolidate and simplify OpenAI provider and provider concepts

* Clean up assistant streaming

* Improve assistant message orchestration logic

* Clean up "thinking" UI interactions

* Remove stale class

* Regenerate VCR test responses

app/models/user_message.rb

@@ -14,9 +14,4 @@ class UserMessage < Message
   def request_response
     chat.ask_assistant(self)
   end
-
-  private
-    def broadcast?
-      true
-    end
 end