Doesn't correctly convert 0 XAH to sto using `float_sto`

[tequdev]

Issue Description

Steps to Reproduce

#include "hookapi.h"

int64_t hook(uint32_t reserved) {
  _g(1, 1);

  uint8_t buf[8];

   uint64_t float1 = 0;

  // 1000000000000000 * 10^*(-16)  =  0.1
  // uint64_t float1 = 6071852297695428608; // underflow case

  int64_t result = float_sto(SBUF(buf), 0, 0, 0, 0, float1, 0);

  trace_num(SBUF("result"), (int64_t)result); // 0
  trace(SBUF("buf"), SBUF(buf), 1);  // 0000000000000000

  return accept(0, 0, 0);
}

Expected Result

result = 8
buf = 4000000000000000

or

result = <some error code>
buf = 0000000000000000

Actual Result

result = 0
buf = 0000000000000000

Environment

2025.7.9-release+1951

[tequdev]

xahaud/src/ripple/app/hook/impl/applyHook.cpp

Line 5387 in 775fb3a

return 0;