Use offsetof for struct field pointer calculation instead of implicit layout assumption

[Copilot]

Addresses review feedback on PR #34351 regarding fragile pointer casting that assumed specific memory layout.

Changes

• Replaced implicit cast with explicit offset calculation: The original code cast &pList->numWhiteLists directly to SDateTimeWhiteList*, relying on numWhiteLists being the first field. This is fragile and breaks if struct layout changes.

• Introduced helper function uvListIsTimeInWhiteList: Encapsulates the pointer arithmetic using offsetof() macro for compile-time safety.

• Added robustness improvements: Additional null checks, explicit ownership transfer semantics in uvDataTimeWhiteListAdd(), and proper cleanup in uvDataTimeWhiteListDestroy().

Before

SDateTimeWhiteList* p = (SDateTimeWhiteList*)&pList->numWhiteLists;
int64_t now = taosGetTimestampSec();
if (isTimeInDateTimeWhiteList(p, now)) {
  return true;
}

After

const SDateTimeWhiteList* pList =
    (const SDateTimeWhiteList*)((const char*)pWhiteList + offsetof(SUserDateTimeWhiteList, numWhiteLists));
return isTimeInDateTimeWhiteList(pList, now);

The offsetof approach makes the struct layout dependency explicit and will cause compile-time errors if the struct definition changes incompatibly.

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