refactor(playground): extract non-stream response handling

- move chat completion response choice handling into message streaming utilities
- keep the chat handler focused on request lifecycle and error routing

web/default/src/features/playground/hooks/use-chat-handler.ts

@@ -26,8 +26,9 @@ import {
   updateAssistantMessageWithError,
   updateLastAssistantMessage,
   parseRequestErrorDetails,
-  applyChatCompletionChoice,
+  applyChatCompletionResponse,
   completeAssistantMessage,
+  hasChatCompletionChoice,
   isAssistantMessageFinal,
   isAssistantMessagePending,
 } from '../lib'
@@ -137,16 +138,20 @@ export function useChatHandler({
         )
         if (abortController.signal.aborted) return
 
-        const choice = response.choices?.[0]
-        if (!choice) {
+        if (!hasChatCompletionChoice(response)) {
           handleStreamError(ERROR_MESSAGES.API_REQUEST_ERROR)
           return
         }
 
         onMessageUpdate((prev) =>
-          updateLastAssistantMessage(prev, (message) =>
-            applyChatCompletionChoice(message, choice)
-          )
+          updateLastAssistantMessage(prev, (message) => {
+            const updatedMessage = applyChatCompletionResponse(
+              message,
+              response
+            )
+
+            return updatedMessage ?? message
+          })
         )
       } catch (error: unknown) {
         if (abortController.signal.aborted) return

web/default/src/features/playground/lib/message-streaming-utils.ts

@@ -125,6 +125,12 @@ export function isAssistantMessagePending(message: Message): boolean {
 
 type ChatCompletionChoice = ChatCompletionResponse['choices'][number]
 
+export function hasChatCompletionChoice(
+  response: ChatCompletionResponse
+): boolean {
+  return Boolean(response.choices?.[0])
+}
+
 export function applyChatCompletionChoice(
   message: Message,
   choice: ChatCompletionChoice
@@ -138,6 +144,19 @@ export function applyChatCompletionChoice(
   }
 }
 
+export function applyChatCompletionResponse(
+  message: Message,
+  response: ChatCompletionResponse
+): Message | null {
+  const choice = response.choices?.[0]
+
+  if (!choice) {
+    return null
+  }
+
+  return applyChatCompletionChoice(message, choice)
+}
+
 /**
  * Sanitize messages loaded from storage.
  * Converts stuck loading/streaming messages to stable state.