Wire the Windows MXC sandbox into command execution (#45176)

## What changed

- Add explicit MXC backend selection and carry its identity through exec-server process reporting and sandbox violation classification.
- Launch MXC through the Codex executable with the effective permission profile and command environment.
- Reject exec-server MXC requests when native MXC is unavailable or when they request a TTY, an `arg0` override, or managed networking. Reject private desktop isolation during MXC preparation.
- Allow an explicitly empty child environment and avoid exposing request payload values in launcher decode errors.

## Testing

Add coverage for sandbox selection and unsupported-request rejection, plus Windows RPC tests for stdin writes and temporary-directory permissions derived from the command environment. Native MXC tests skip when MXC is unavailable.

GitOrigin-RevId: 3626ff0f9ad7f9b812ce09b68c31ea9a5a9c72b1

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

@@ -4907,7 +4907,7 @@ impl ThreadRequestProcessor {
                         serde_json::json!("unelevated"),
                     );
                 }
-                WindowsSandboxLevel::Disabled => {}
+                WindowsSandboxLevel::Disabled | WindowsSandboxLevel::Mxc => {}
             }
         }
         let request_overrides = if cli_overrides.is_empty() {