Prevent cancelled CbfClient.update() calls from losing queued blocks

`CbfClient.update()` directly exposes Kyoto’s non cancel safe update future. When a foreign coroutine is cancelled UniFFI can drop that future and lose queued block state.

Start at the CbfClient.update() entry point and inspect how its Kyoto update future is exposed through UniFFI and reacts to cancellation from a foreign coroutine. Done means cancelling the call no longer drops queued block state; verify this with the relevant existing tests or a focused regression test if the repository provides a suitable test entry point.

Written by the indexing model from the issue text.