Always use private desktops for legacy Windows sandboxes (#46554)

## What changed

- Remove the private-desktop opt-out from elevated and unelevated Windows sandbox launches.
- Remove `windows.sandbox_private_desktop` and its managed requirement and API fields. Warn users to remove the obsolete setting.
- Require a private desktop name when launching through the Windows sandbox wrapper and command runner.

## Testing

Add coverage for the obsolete-setting migration warning and update wrapper tests to verify a live private desktop is passed and a missing desktop name is rejected.

GitOrigin-RevId: c7135f8d211aac8d812180691c2c1e433d77cde4

codex-rs/app-server/src/command_exec.rs

@@ -719,7 +719,6 @@ mod tests {
             SandboxType::WindowsRestrictedToken,
             vec![cwd],
             WindowsSandboxLevel::Disabled,
-            /*windows_sandbox_private_desktop*/ false,
             PermissionProfile::read_only(),
             /*arg0*/ None,
         )
@@ -837,7 +836,6 @@ mod tests {
                     SandboxType::None,
                     vec![cwd.clone()],
                     WindowsSandboxLevel::Disabled,
-                    /*windows_sandbox_private_desktop*/ false,
                     PermissionProfile::read_only(),
                     /*arg0*/ None,
                 ),
@@ -931,7 +929,6 @@ mod tests {
                     SandboxType::None,
                     vec![cwd],
                     WindowsSandboxLevel::Disabled,
-                    /*windows_sandbox_private_desktop*/ false,
                     PermissionProfile::read_only(),
                     /*arg0*/ None,
                 ),