test(drvfs): add O_APPEND ftruncate coverage, skip on virtiofs

[Ben Hillis (benhillis)]

Summary

Adds an ftruncate-on-O_APPEND regression check to the drvfs Basic unit test. This scenario (open(O_APPEND | O_WRONLY) → write → ftruncate) was previously uncovered, and it currently fails on virtiofs with EACCES, breaking real-world workloads (see #40987).

Root cause is in the Windows lxutil file server (wsldevicehost): opening with O_APPEND strips FILE_WRITE_DATA from the NT access mask to enforce append-only writes, but the truncate path is issued on that same handle, so SetEndOfFile is denied. plan9 is unaffected because it doesn't map append semantics to a reduced NT access mask.

Changes

• test/linux/unit_tests/drvfs.c — new ftruncate-on-O_APPEND coverage in DrvFsTestBasic. It runs on plan9 and is skipped on virtiofs (gated on g_LxtFsInfo.FsType == LxtFsTypeVirtioFs) with a LxtLogInfo note, since the backend fix hasn't shipped yet.
• packages.config — TODO above Microsoft.WSL.DeviceHost to bump the package to a build containing the virtiofs fix and then remove the virtiofs skip.

Validation

Built Debug and ran locally:

Test	Transport	Result
WSL2VirtioFs::DrvFs	virtiofs	Passed (ftruncate check skipped, logs issue reference)
WSL2Plan9::DrvFs	plan9	Passed (ftruncate check runs)

Without the skip, WSL2VirtioFs::DrvFs fails with ftruncate(Fd, 0) failed: errno 13 (Permission denied), confirming the test reproduces the bug. clang-format clean.

Once an updated Microsoft.WSL.DeviceHost with the fix is consumed, the virtiofs skip should be removed so the check runs on both transports.

[Copilot]

Pull request overview

This PR strengthens drvfs Linux unit test coverage by adding a regression check for ftruncate() behavior on a file descriptor opened with O_APPEND, while temporarily skipping the check on virtiofs where the underlying backend bug is still present.

Changes:

• Extend DrvFsTestBasic to validate that ftruncate() succeeds on an O_APPEND | O_WRONLY descriptor (and that the resulting file size matches expectations).
• Skip the new check on virtiofs (with an informational log) to avoid known failures until the backend fix ships.
• Add a TODO note in packages.config to bump Microsoft.WSL.DeviceHost and remove the virtiofs skip once the fix is available.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/linux/unit_tests/drvfs.c	Adds O_APPEND + ftruncate regression coverage in DrvFsTestBasic, with a virtiofs skip gate and log message.
packages.config	Documents the dependency bump TODO needed to remove the virtiofs skip once the fix is consumed.