Separate executor sandbox selection from Windows sandbox levels (#45730)

## Why

MXC is a sandbox implementation, not a restricted-token sandbox level. Executor requests need to represent that choice separately from `WindowsSandboxLevel`.

## What changed

- Introduce `WindowsSandboxSelection` for executor sandbox contexts and remove `Mxc` from `WindowsSandboxLevel`.
- Preserve the `windowsSandboxLevel` wire field and its serialized values for compatibility.
- Share sandbox selection between executor process launches and filesystem helpers, and use the new selection in capability discovery and skill reads.
- Disable Windows sandbox selection for executor paths that do not use Windows path conventions.

## Testing

Extend coverage for MXC wire serialization, Windows skill-read sandbox checks, and capability discovery with distinct permissions. Exercise remote filesystem write restrictions with both restricted-token and MXC sandboxes, including rejection when native MXC is unavailable.

GitOrigin-RevId: 266211377bcb138a0dc75861e9ff2225fa37a53d

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -4931,7 +4931,7 @@ impl ThreadRequestProcessor {
                         serde_json::json!("unelevated"),
                     );
                 }
-                WindowsSandboxLevel::Disabled | WindowsSandboxLevel::Mxc => {}
+                WindowsSandboxLevel::Disabled => {}
             }
         }
         let request_overrides = if cli_overrides.is_empty() {