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/provider/llm_concept.rb

@@ -0,0 +1,12 @@
+module Provider::LlmConcept
+  extend ActiveSupport::Concern
+
+  ChatMessage = Data.define(:id, :output_text)
+  ChatStreamChunk = Data.define(:type, :data)
+  ChatResponse = Data.define(:id, :model, :messages, :function_requests)
+  ChatFunctionRequest = Data.define(:id, :call_id, :function_name, :function_args)
+
+  def chat_response(prompt, model:, instructions: nil, functions: [], function_results: [], streamer: nil, previous_response_id: nil)
+    raise NotImplementedError, "Subclasses must implement #chat_response"
+  end
+end