Fix use-after-free in virtiofs request worker thread

[OneBlue]

Summary of the Pull Request

This change solves a potential use-after-free caused by the lack of synchronization with threads that are spawned to handle virtiofs requests from the guest.

To solve this, this change moves that method to use overlapped IO and handle both the accepts() and the requests with a single thread

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

This PR addresses a potential use-after-free in the VirtioFS request handling path by moving request processing to an overlapped I/O model that can accept connections and process requests within a single worker thread. It also refactors shared accept logic to return an accepted socket directly, and adds a regression-style test to stress multiple VirtioFS-backed DrvFs mounts.

Changes:

• Refactor VirtioFS request worker to use io::MultiHandleWait with overlapped accept/read/write handles instead of spawning a thread per request.
• Change socket::CancellableAccept to return std::optional<wil::unique_socket> and introduce a reusable io::AcceptHandle implementation.
• Add a DrvFs test that mounts many VirtioFS shares in a loop to validate stability.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/windows/DrvFsTests.cpp	Adds a stress test that mounts many VirtioFS DrvFs shares and validates access/options.
src/windows/wslrelay/main.cpp	Updates to new CancellableAccept return type (optional accepted socket).
src/windows/wslrelay/localhost.cpp	Updates accept loop to new CancellableAccept return type and moves socket out of optional.
src/windows/service/exe/WslCoreVm.h	Adds ProcessVirtioFsRequest helper declaration for the new VirtioFS worker flow.
src/windows/service/exe/WslCoreVm.cpp	Reworks VirtioFS worker to single-threaded overlapped accept/read/write and factors request processing into ProcessVirtioFsRequest.
src/windows/common/socket.hpp	Updates CancellableAccept signature to return an optional accepted socket.
src/windows/common/socket.cpp	Implements new CancellableAccept using io::AcceptHandle and returns the accepted socket via std::optional.
src/windows/common/hvsocket.cpp	Delegates HvSocket CancellableAccept to the updated generic socket accept helper.
src/windows/common/HandleIO.h	Replaces SingleAcceptHandle with reusable AcceptHandle that can accept once or repeatedly and owns the accepted socket.
src/windows/common/HandleIO.cpp	Implements AcceptHandle (socket creation, accept completion, accept-context update, cancellation handling).

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.