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/exec-server-protocol/src/protocol.rs

@@ -152,6 +152,9 @@ pub struct EnvironmentCapabilities {
     /// Whether shell state can be cached and restored entirely inside the executor.
     #[serde(default)]
     pub shell_snapshot_v2: bool,
+    /// Whether requests may explicitly select the MXC Windows sandbox backend.
+    #[serde(default)]
+    pub windows_mxc: bool,
 }
 
 /// Status returned by an initialized exec-server connection.
@@ -243,6 +246,7 @@ impl EnvironmentInfo {
                 http_header_env_vars: true,
                 sandboxed_file_streaming: true,
                 shell_snapshot_v2: cfg!(unix),
+                windows_mxc: false,
             },
         }
     }
@@ -358,6 +362,7 @@ pub enum ProcessSandboxType {
     MacosSeatbelt,
     LinuxSeccomp,
     WindowsRestrictedToken,
+    WindowsMxc,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
@@ -1057,6 +1062,7 @@ mod tests {
                 http_header_env_vars: false,
                 sandboxed_file_streaming: false,
                 shell_snapshot_v2: false,
+                windows_mxc: false,
             }
         );
     }
@@ -1078,6 +1084,7 @@ mod tests {
                 "httpHeaderEnvVars": false,
                 "sandboxedFileStreaming": false,
                 "shellSnapshotV2": false,
+                "windowsMxc": false,
             },
         });
         let info: EnvironmentInfo = serde_json::from_value(expected.clone())