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/schema/json/codex_app_server_protocol.schemas.json

@@ -10032,12 +10032,6 @@
               "string",
               "null"
             ]
-          },
-          "windowsSandboxPrivateDesktop": {
-            "type": [
-              "boolean",
-              "null"
-            ]
           }
         },
         "type": "object"

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -5912,12 +5912,6 @@
             "string",
             "null"
           ]
-        },
-        "windowsSandboxPrivateDesktop": {
-          "type": [
-            "boolean",
-            "null"
-          ]
         }
       },
       "type": "object"

codex-rs/app-server-protocol/schema/json/v2/ConfigRequirementsReadResponse.json

@@ -599,12 +599,6 @@
             "string",
             "null"
           ]
-        },
-        "windowsSandboxPrivateDesktop": {
-          "type": [
-            "boolean",
-            "null"
-          ]
         }
       },
       "type": "object"