fix(string_util): add missing <cstdint> include for uint64_t

Fixes #856. On some platforms (e.g., Fedora 43 with recent GCC),
<cstdint> is not transitively included, causing a build failure
because uint64_t is not declared in string_util.h.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Chessing234

src/include/common/util/string_util.h

@@ -12,6 +12,7 @@
 
 #pragma once
 
+#include <cstdint>
 #include <string>
 #include <vector>