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-protocol/src/protocol/v2/config.rs

@@ -456,7 +456,6 @@ pub struct ConfigRequirements {
     pub check_for_update_on_startup: Option<bool>,
     pub allow_login_shell: Option<bool>,
     pub feedback: Option<FeedbackRequirements>,
-    pub windows_sandbox_private_desktop: Option<bool>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -2159,7 +2159,6 @@ fn config_requirements_granular_allowed_approval_policy_is_marked_experimental()
             check_for_update_on_startup: None,
             allow_login_shell: None,
             feedback: None,
-            windows_sandbox_private_desktop: None,
         });
 
     assert_eq!(reason, Some("askForApproval.granular"));