Fix DateTimeUtils UTC string semantics and remove deprecated substr usage

[Copilot]

DateTimeUtils.UTCToString was returning local-time output (toTimeString()), which contradicted its UTC naming and made output timezone-dependent across environments. The DateTimeUtils UTC parsing methods also used legacy substr().

• UTC output correctness

  • UTCToString now returns a UTC-stable HH:MM:SS value derived from toISOString() instead of local-time toTimeString().

• Deprecated API cleanup

  • Replaced substr() with substring() in:
    • UTCToString
    • UTCDateTimeToString

• Focused coverage for UTC formatting paths

  • Added lib/DateTimeUtils.test.ts to cover:
    • UTCToString('1530') -> '15:30:00'
    • UTCDateTimeToString handling for both HHMM and HHMMSS

public static UTCToString(UTCString: string) {
  let utcDate = new Date();
  utcDate.setUTCHours(
    +UTCString.substring(0, 2),
    +UTCString.substring(2, 4),
    0,
  );
  return utcDate.toISOString().substring(11, 19);
}