feat(react): add newThreadId option to useStream for optimistic UI

Add optional newThreadId parameter to useStream hook that allows specifying
a thread ID for new thread creation while keeping threadId null. This enables
optimistic UI patterns where developers need to know the thread ID beforehand
for routing/navigation without causing 404 errors from attempting to fetch
non-existent thread history.

Usage:
- Set threadId: null and newThreadId: "predetermined-id"
- Submit message to create thread with specified ID
- Use onThreadId callback to update threadId after creation

This solves the UX problem of having to await thread creation before
enabling optimistic navigation to e.g. /[threadId] routes.

libs/sdk-js/src/react/stream.tsx

@@ -516,6 +516,13 @@ export interface UseStreamOptions<
    */
   threadId?: string | null;
 
+  /**
+   * The ID to use when creating a new thread. When provided, this ID will be used
+   * for thread creation when threadId is null. This enables optimistic UI updates
+   * where you know the thread ID before the thread is actually created.
+   */
+  newThreadId?: string;
+
   /**
    * Callback that is called when the thread ID is updated (ie when a new thread is created).
    */
@@ -1013,7 +1020,9 @@ export function useStream<
 
       let usableThreadId = threadId;
       if (!usableThreadId) {
-        const thread = await client.threads.create();
+        const thread = await client.threads.create({
+          threadId: options.newThreadId,
+        });
         onThreadId(thread.thread_id);
         usableThreadId = thread.thread_id;
       }