## 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