Fix: mark useAssistant as in progress for append/submitMessage. (#1423)

Author: lgrammel

.changeset/strong-countries-clap.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+Fix: mark useAssistant as in progress for append/submitMessage.

packages/core/react/use-assistant.ts

@@ -141,6 +141,8 @@ export function useAssistant({
       data?: Record<string, string>;
     },
   ) => {
+    setStatus('in_progress');
+
     setMessages(messages => [
       ...messages,
       {