[roll] Roll fuchsia ffx: introduce timeouts in fastboot usb discovery

Prevent slow-responding or hung USB devices from blocking ffx
fastboot discovery by introducing a lazy-evaluation mechanism
for serial numbers and parallelizing device scans with a timeout.

Previously, walking the USB bus was sequential and synchronous:
- usb_rs::enumerate_devices eagerly read serial descriptors from
  sysfs, blocking the executor if a device was unresponsive.
- usb_fastboot_discovery sequentialized scans on the main async
  thread, compounding the delays.

This change:
1. Converts usb_rs serial retrieval to lazy evaluation using OnceLock.
   `enumerate_devices` now completes near-instantly.
2. Parallelizes target checks in usb_fastboot_discovery by spawning
   them concurrently.
3. Dispatches the blocking USB descriptor read operations to a helper
   thread via `fuchsia_async::unblock`.
4. Limits each device check to a 1-second timeout via `.on_timeout()`.

This ensures a single bad machine spirit does not stall the entire ffx
target enumeration pipeline.

      note it is skipped.

Test: Ran `ffx target list` with an inentionally slow usb device and
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1623116
Original-Original-Revision: 7fb9e18b1d9f030b418fc919a9280a701c0f9f67
GitOrigin-RevId: fa109fe16c04252eead76957e71f55af9ab591a9
Change-Id: Ia39605d0548bf1ca12b4a09cbc6a4cbb65b7a1cc
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1622396
Cr-Commit-Position: refs/heads/main@{#193465}