fix(websocket): use debug level for http.Server errors

https://github.com/libp2p/go-libp2p/pull/3364 migrated from zap to slog
but accidentally changed the log level for http.Server.ErrorLog from
implicit INFO to explicit ERROR. This caused client EOF and TLS handshake
errors to spam error logs and stdout in apps which log only ERROR by
default.

These http.Server errors (client EOFs, TLS handshake failures from clients
with naive TLS implementations, connection timeouts from clients that abort
early) are normal operational noise, not actual server errors. Using
LevelDebug:
- matches semantic meaning (similar to existing connection timeout logs)
- respects user's configured threshold (default ERROR filters them out)
- allows users to enable for debugging via log level configuration

Fixes https://github.com/ipfs/kubo/issues/11027
Fixes https://github.com/ipfs/kubo/issues/11033

p2p/transport/websocket/listener.go

@@ -126,8 +126,10 @@ func newListener(a ma.Multiaddr, tlsConf *tls.Config, sharedTcp *tcpreuse.ConnMg
 		},
 	}
 	ln.server = http.Server{
-		Handler:     ln,
-		ErrorLog:    slog.NewLogLogger(log.Handler(), slog.LevelError),
+		Handler: ln,
+		// Use LevelDebug for http.Server errors (TLS handshake failures, connection issues).
+		// These are operational noise from misbehaving/buggy remote clients, not server errors.
+		ErrorLog:    slog.NewLogLogger(log.Handler(), slog.LevelDebug),
 		ConnContext: ln.ConnContext,
 		TLSConfig:   tlsConf,
 	}