fix(sdk): clear messages on hydrate(null) with a pending interrupt (#2691)

## Summary

Teardown awaited the paused root pump, so threadId went null while the
old conversation stayed on screen. Reset the snapshot first.

- Starting a new chat (`threadId` → undefined) on a reused `useStream`
left the previous thread's messages on screen when that thread was
sitting on a pending interrupt. The snapshot now clears before teardown
finishes waiting on the paused pump.