Clean up Windows action process trees

[alex-the-third]

On Windows, actions may exit while leaving subprocesses running. Those subprocesses can retain their working directory or open files inside the action root, causing the executor to fail when removing it after the action completes.

This implements the approach proposed in #245 and follows Bazel's native Windows launcher.

Implementation

Each action process is placed in its own non-breakaway Windows Job Object configured with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE.

The root process is created suspended, assigned to the job, and only then resumed. This prevents it from spawning descendants before job assignment. Because Go's os/exec closes the primary thread handle before cmd.Start() returns, the implementation locates the root process's threads by PID and resumes them through the Windows API.

When the root process exits, any remaining processes in the job are terminated. The runner then waits for JOB_OBJECT_MSG_ACTIVE_PROCESS_ZERO before returning, ensuring that descendants no longer hold files or directories inside the action root.

Command cancellation terminates the entire job instead of only the root process. Startup cancellation is serialized with process resumption so cancellation cannot terminate the suspended process between the cancellation check and resume.

Non-Windows behavior remains unchanged.

Testing

Added Windows tests covering:

• A parent that exits while leaving a child holding a file open inside the action root.
• Cancellation of a running parent and descendant.
• The race between the startup cancellation check and resuming the suspended process.
• Immediate removal of the action root after Run() returns.

Fixes #245