fix(react): refresh transport across renders in useChat

[naumanAhmed3]

What

useChat previously captured transport (and any body / headers / prepareSendMessagesRequest baked into it) on the very first render via the initial new Chat(...) call, so an inline transport closing over component state would forever send the value from that first render. The community report in #7819 shows the most common shape:

const [subagent, setSubagent] = useState('todos-agent');

useChat({
  transport: new DefaultChatTransport({
    body: { subagent },          // ← always 'todos-agent', even after setSubagent(...)
  }),
});

The other callback options (onFinish, onError, onToolCall, ...) already use a callbacksRef pattern to stay fresh across renders. This PR extends the same pattern to transport:

1. Keep the user-supplied transport in a userTransportRef that gets refreshed on every render.
2. Create a stable delegating ChatTransport proxy once. Its identity never changes, so Chat happily keeps it across renders, but each sendMessages / reconnectToStream call reads through to whatever the most-recent user transport is.

If the user does not supply a transport, the ref stays unset and Chat falls back to its built-in default (no behavior change for that path).

Why this approach

• Symmetric with the existing callbacksRef pattern for onFinish / onError / onToolCall already present in this hook — least surprising for readers.
• No Chat API change. transport is private readonly on AbstractChat and stays that way; the proxy lives on the React side only.
• Works for both shapes users report:
  • inline new DefaultChatTransport({ body: { state } }) recreated each render
  • a custom transport instance that itself reads from React/Zustand/etc. — both go through the same userTransportRef.

Tests

Adds one regression test (describe('transport state stays fresh across renders (#7819)')) that mirrors the exact issue body:

• Renders a component with [subagent, setSubagent] = useState('todos-agent')
• Passes that into body: { subagent } on a DefaultChatTransport constructed inline
• Clicks a button to set the state to 'research-agent'
• Clicks send and asserts the request body's subagent is 'research-agent', not the stale 'todos-agent'

Before this fix the assertion would fail with 'todos-agent'.

Notes

• I don't have a local Node toolchain to run vitest myself; relying on CI to confirm the regression test and the rest of the suite still pass. Happy to push fixups for any maintainer feedback.

Closes #7819.