Reuse private desktops across Windows sandbox commands (#40672)

## Why

Private desktops need to remain available across command-runner exits and idle
gaps.

## What changed

- Keep parent-owned private desktops alive and reuse them only when the sandbox
  account and effective permissions match.
- Pass the selected desktop name to elevated command runners, which validate and
  open the existing desktop instead of creating their own.
- Apply the same policy-aware reuse to the legacy Windows sandbox path.

## Testing

Add Windows tests for desktop-name validation, lifetime and concurrent reuse,
and separation when effective permissions differ.

GitOrigin-RevId: 2df8b1c31ff22d72549f2ec3065cd80ea1dd0c6a