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/openai/chat_stream_parser.rb

@@ -0,0 +1,28 @@
+class Provider::Openai::ChatStreamParser
+  Error = Class.new(StandardError)
+
+  def initialize(object)
+    @object = object
+  end
+
+  def parsed
+    type = object.dig("type")
+
+    case type
+    when "response.output_text.delta", "response.refusal.delta"
+      Chunk.new(type: "output_text", data: object.dig("delta"))
+    when "response.completed"
+      raw_response = object.dig("response")
+      Chunk.new(type: "response", data: parse_response(raw_response))
+    end
+  end
+
+  private
+    attr_reader :object
+
+    Chunk = Provider::LlmConcept::ChatStreamChunk
+
+    def parse_response(response)
+      Provider::Openai::ChatParser.new(response).parsed
+    end
+end