fix: await convertToModelMessages in docs chat API route (#1526)

convertToModelMessages is async but was not being awaited, so a Promise
was passed to streamText instead of the resolved ModelMessage array.
This caused Zod validation to fail on every chat request with:
"Invalid prompt: The messages do not match the ModelMessage[] schema."

Author: johnlindquist

docs/app/api/chat/route.ts

@@ -81,10 +81,10 @@ User question: ${userQuestion}`,
 
     const stream = createUIMessageStream({
       originalMessages: messages,
-      execute: ({ writer }) => {
+      execute: async ({ writer }) => {
         const result = streamText({
           model: 'openai/gpt-4.1-mini',
-          messages: convertToModelMessages(processedMessages),
+          messages: await convertToModelMessages(processedMessages),
           stopWhen: stepCountIs(10),
           tools: createTools(writer),
           system: createSystemPrompt(currentRoute),