Add a Windows MXC sandbox opt-out (#51547)

## What changed

Add `windows.allow_mxc` to configuration and its JSON schema. Setting it to `false` blocks automatic MXC selection even when `features.prefer_mxc` is enabled, and rejects explicit `windows.sandbox = "mxc"` configuration with a clear error. Omitting the setting preserves existing behavior.

## Testing

Extend configuration tests to cover disabling automatic MXC selection and rejecting explicit MXC configuration when `windows.allow_mxc = false`.

GitOrigin-RevId: bf7c0648cfcb3cc1d0dc41b9faffcb8078672780

codex-rs/config/src/types.rs

@@ -194,6 +194,8 @@ pub enum WindowsSandboxModeToml {
 #[schemars(deny_unknown_fields)]
 pub struct WindowsToml {
     pub sandbox: Option<WindowsSandboxModeToml>,
+    /// False blocks both explicit MXC configuration and automatic selection.
+    pub allow_mxc: Option<bool>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Copy, Clone, PartialEq, JsonSchema)]