fix(react): prevent Chat recreation when id is undefined

[zhsj0089944]

Problem

Passing id: undefined to useChat causes the internal Chat instance to be recreated on every render. Sent messages and in-flight streams disappear immediately.

This is easy to hit because id={conversationId ?? undefined} is a common pattern.

Root cause

In useChat:

const shouldRecreateChat =
    ('chat' in options && options.chat !== chatRef.current) ||
    ('id' in options && chatRef.current.id !== options.id);

When id: undefined is passed:

1. 'id' in options is true (key exists even though value is undefined)
2. chatRef.current.id is an auto-generated UUID
3. uuid !== undefined is always true

So shouldRecreateChat stays true and a fresh Chat is created every render.

Fix

Add options.id !== undefined check so that id: undefined behaves the same as omitting id:

  const shouldRecreateChat =
    ('chat' in options && options.chat !== chatRef.current) ||
-   ('id' in options && chatRef.current.id !== options.id);
+   ('id' in options && options.id !== undefined && chatRef.current.id !== options.id);

Fixes #16226