Replace redundant test disk implementations with RAM disk backend

[Copilot]

Now that disklayer_ram supports custom sector sizes (RamDiskLayer::new_with_sector_size, ram_disk_with_sector_size), two custom DiskIo test implementations that existed as workarounds can be removed.

virtio_blk — remove TestDisk4K

TestDisk4K was explicitly documented as a workaround for non-512-byte sector testing. Replaced with disklayer_ram::ram_disk_with_sector_size(). The with_discard() variant is also eliminated since the RAM disk already returns UnmapBehavior::Unspecified.

Removed dev-dependencies: parking_lot. Removed unused imports: DiskError, DiskIo, Inspect, MemoryRead, MemoryWrite.

scsidisk/scsidvd — remove local TestDisk

The test module had its own TestDisk that pre-seeded sector data for read tests. Replaced with a RAM disk, pre-filled synchronously via now_or_never() before constructing SimpleScsiDvd:

fn new_scsi_dvd(sector_size: u32, sector_count: u64) -> SimpleScsiDvd {
    let disk = disklayer_ram::ram_disk_with_sector_size(disk_size, false, sector_size).unwrap();
    // Pre-fill pattern data synchronously — RAM disk ops never block
    disk.write_vectored(&buffers.buffer(&mem), 0, false)
        .now_or_never()
        .expect("RAM disk write should complete synchronously")
        .unwrap();
    SimpleScsiDvd::new(Some(disk))
}

The read_only parameter was dropped (all callers passed true; the underlying disk must be writable for pre-fill, and DVD write protection is enforced at the SCSI layer).

Added disklayer_ram to scsidisk dev-dependencies.

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Not changed: scsidisk/tests/test_helpers.rs TestDisk — retained because it provides FUA tracking, dynamic sector-count mutation for resize tests, and asymmetric physical/logical sector sizes, none of which the RAM disk backend exposes.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Redundant test disk implementations</issue_title>
<issue_description>Now that #2927 is done, the RAM disk backend can handle multiple sector sizes. Some test code around the tree was using its own mock disk implementation to work around this missing functionality. Search for such code and replace it with the RAM disk backend.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Redundant test disk implementations #2994

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