Add tests for Unicode path component decoding in decode_path_component function

httplib.h

@@ -9292,7 +9292,8 @@ inline std::string decode_path_component(const std::string &component) {
         // Unicode %uXXXX encoding
         auto val = 0;
         if (detail::from_hex_to_i(component, i + 2, 4, val)) {
-          // 4 digits Unicode codes
+          // 4 digits Unicode codes: val is 0x0000-0xFFFF (from 4 hex digits),
+          // so to_utf8 writes at most 3 bytes. buff[4] is safe.
           char buff[4];
           size_t len = detail::to_utf8(val, buff);
           if (len > 0) { result.append(buff, len); }