[roll] Roll fuchsia [netstack3] Remove cancel oneshot from rolling pcap Attached state

Simplify `RollingCaptureState::Attached` by removing the `cancel`
sender.

Instead of creating a `oneshot` channel at startup, we now use a
dynamic `Either` future in the `serve_rolling_packet_capture` loop.

The loop starts with a `pending()` future (which never resolves)
when in the `Attached` state. When `Detach` is called, we create
the channel on the fly, store the sender in the `Detached` state,
and replace the local future with the `Receiver`.

This avoids creating unused channels at startup and simplifies the
`Attached` state representation.

Additionally, use `now_or_never()` on the `Either` future to check
for takeover signals in the disconnected phase, avoiding the need
to reach into the `Fuse` wrapper.

Original-Original-Bug: 485274945
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1671616
Original-Original-Revision: 068b1cd0dd09eb3cab06d3e4c100321257738843
GitOrigin-RevId: f87c43946fd384ac7bc2630e99137fff2db11d14
Change-Id: Ica1b2533125b367a6f5458c2db4ab8857fb91024
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1682615
Cr-Commit-Position: refs/heads/main@{#195698}