Resolve CI failures from rustfmt/clippy regressions

[Copilot]

CI checks were failing due to rustfmt drift and clippy warnings treated as errors. This update aligns formatting and addresses the specific lints blocking the pipeline.

• Clippy fixes: collapse nested conditionals, add Default for VlanStack, and use std::io::Error::other where appropriate.
• Formatting alignment: apply rustfmt updates to touched modules to match enforced style.

if let DrainMode::ShutdownOnly = mode && !matches!(event, WorkerEvent::Shutdown(_)) {
    return Ok(());
}