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/assistant_message.rb

@@ -5,7 +5,8 @@ class AssistantMessage < Message
     "assistant"
   end
 
-  def broadcast?
-    true
+  def append_text!(text)
+    self.content += text
+    save!
   end
 end