test(gateway): cover visible iface alias help

The MVP test guide relies on --iface being both accepted and discoverable from
lanparty-gateway --help. The parser test already covered acceptance, but it did
not protect the rendered help text.

Render the Clap long help in a unit test and assert that the visible alias is
advertised. This keeps the handoff guide and binary self-documentation aligned.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-gateway iface_alias
- cargo run -p lanparty-gateway -- --help
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check

Refs: MVP gateway test handoff

crates/lanparty-gateway/src/lib.rs

@@ -1045,6 +1045,7 @@ mod tests {
     use std::time::Duration;
 
     use bytes::Bytes;
+    use clap::CommandFactory;
     use lanparty_net::DEFAULT_RELAY_PORT;
     use quinn::{ServerConfig, TransportConfig, crypto::rustls::QuicServerConfig};
     use rustls::pki_types::{PrivateKeyDer, PrivatePkcs8KeyDer};
@@ -1121,6 +1122,19 @@ mod tests {
         assert_eq!(args.interface, "eth0");
     }
 
+    #[test]
+    fn shows_iface_alias_in_help() {
+        let mut command = GatewayArgs::command();
+        let mut help = Vec::new();
+        command.write_long_help(&mut help).unwrap();
+        let help = String::from_utf8(help).unwrap();
+
+        assert!(
+            help.contains("[aliases: --iface]"),
+            "gateway help should advertise --iface alias:\n{help}"
+        );
+    }
+
     #[tokio::test]
     async fn connects_to_relay_control_stream_as_gateway() {
         let (server_config, certificate) = test_server_config();