* Update Microsoft.WSL.Kernel to 6.18.26.1-1

* Update tests for Linux 6.18 kernel behavior changes

Adjust eventfd size validation, lxtfs writev, and mount option
format expectations to match 6.18 kernel behavior.

* Update test patterns for new kernel /proc/mounts cache format

The kernel now outputs cache=0x5 (hex) instead of cache=5 (decimal) in
/proc/mounts for 9p filesystems. Update the ExpectMount patterns in
WSLCTests::WindowsMounts and WSLCTests::GPU to match the new format.

---------

Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…) (#40532)

HCS fails with E_ACCESSDENIED when starting a VM whose user-supplied
kernelModules or systemDistro VHDs live somewhere VMWP cannot read
(e.g. under the user profile). Eagerly call HcsGrantVmAccess on those
paths while impersonating the user, before the VM is started.

The grant is best-effort: it requires WRITE_DAC on the file (typically
via ownership), which the impersonated user may lack for VHDs they only
have READ access to (e.g. SYSTEM-owned VHDs reachable via inherited
folder ACLs). Failures are logged via CATCH_LOG; if VMWP truly cannot
read the VHD, StartComputeSystem will still surface a clear
E_ACCESSDENIED.

Adds two regression tests:
- CustomVhdsInUserProfile: VHDs under %TEMP%, exercises the grant path.
- CustomVhdsAccessibleViaInheritedAcls: VHDs in the install dir launched
  as a non-elevated user, exercises the swallowed-grant-failure path.

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>