s3api: add WithOnListen option for server readiness notification

WithOnListen registers a callback invoked once the server is bound and
ready to accept connections. It wraps fiber's existing OnListen hook,
which fires immediately before the first connection is served.

This allows callers to detect readiness deterministically rather than
relying on a fixed sleep or polling a health endpoint.

Example use in tests:

    ready := make(chan struct{})
    srv, _ := s3api.New(..., s3api.WithOnListen(func() { close(ready) }))
    go srv.ServeMultiPort([]string{addr})
    <-ready // blocks until the server is accepting connections

s3api/server.go

@@ -58,6 +58,7 @@ type S3ApiServer struct {
 	routes           []routeMount
 	middlewares      []middlewareMount
 	socketPerm       os.FileMode
+	onListen         func()
 }
 
 type routeMount struct {
@@ -328,6 +329,14 @@ func WithDisableACL() Option {
 	return func(s *S3ApiServer) { s.Router.disableACL = true }
 }
 
+// WithOnListen registers a callback that is invoked once the server is bound
+// and ready to accept requests. It is called from the fiber OnListen hook,
+// which fires immediately before the server begins serving the first
+// connection. Use this instead of a fixed sleep to detect readiness reliably.
+func WithOnListen(fn func()) Option {
+	return func(s *S3ApiServer) { s.onListen = fn }
+}
+
 // ServeMultiPort creates listeners for multiple port specifications and serves
 // on all of them simultaneously. This supports listening on multiple ports and/or
 // addresses (e.g., [":7070", "localhost:8080", "0.0.0.0:9090"]).
@@ -362,6 +371,14 @@ func (sa *S3ApiServer) ServeMultiPort(ports []string) error {
 	// Combine all listeners
 	finalListener := utils.NewMultiListener(listeners...)
 
+	if sa.onListen != nil {
+		fn := sa.onListen
+		sa.app.Hooks().OnListen(func(fiber.ListenData) error {
+			fn()
+			return nil
+		})
+	}
+
 	return sa.app.Listener(finalListener)
 }