internal: remove unused util functions (#871)

The functions util.Sorted and util.KeySlice are unused.

internal/util/errors.go

@@ -0,0 +1,16 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package util
+
+import (
+	"fmt"
+)
+
+// Wrapf wraps *errp with the given formatted message if *errp is not nil.
+func Wrapf(errp *error, format string, args ...any) {
+	if *errp != nil {
+		*errp = fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), *errp)
+	}
+}