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/developer_message.rb

@@ -3,7 +3,8 @@ class DeveloperMessage < Message
     "developer"
   end
 
-  def broadcast?
-    chat.debug_mode?
-  end
+  private
+    def broadcast?
+      chat.debug_mode?
+    end
 end