feat: add reasoning_effort configuration support for Doubao/GPT-5 models (#947)

* feat: Add reasoning effort configuration support

* Add `reasoning_effort` parameter to model config and agent initialization
* Support reasoning effort levels (minimal/low/medium/high) for Doubao/GPT-5 models
* Add UI controls in input box for reasoning effort selection
* Update doubao-seed-1.8 example config with reasoning effort support

Fixes & Cleanup:
* Ensure UTF-8 encoding for file operations
* Remove unused imports

* fix: set reasoning_effort to None for unsupported models

* fix: unit test error

* Update frontend/src/components/workspace/input-box.tsx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/components/workspace/messages/message-list.tsx

@@ -34,20 +34,18 @@ export function MessageList({
   className,
   threadId,
   thread,
-  messagesOverride,
+  messages,
   paddingBottom = 160,
 }: {
   className?: string;
   threadId: string;
   thread: UseStream<AgentThreadState>;
-  /** When set (e.g. from onFinish), use instead of thread.messages so SSE end shows complete state. */
-  messagesOverride?: Message[];
+  messages: Message[];
   paddingBottom?: number;
 }) {
   const { t } = useI18n();
   const rehypePlugins = useRehypeSplitWordsIntoSpans(thread.isLoading);
   const updateSubtask = useUpdateSubtask();
-  const messages = messagesOverride ?? thread.messages;
   if (thread.isThreadLoading) {
     return <MessageListSkeleton />;
   }