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/views/assistant_messages/_assistant_message.html.erb

@@ -17,6 +17,7 @@
 
     <div class="flex items-start mb-6">
       <%= render "chats/ai_avatar" %>
+
       <div class="prose prose--ai-chat"><%= markdown(assistant_message.content) %></div>
     </div>
   <% end %>

app/views/chats/show.html.erb

@@ -23,7 +23,7 @@
         <%= render "chats/thinking_indicator", chat: @chat %>
       <% end %>
 
-      <% if @chat.error.present? %>
+      <% if @chat.error.present? && @chat.needs_assistant_response? %>
         <%= render "chats/error", chat: @chat %>
       <% end %>
     </div>