Function convertDigitsToPersianWord

convertDigitsToPersianWord(num: number): string
Converts a numeric value into its full Persian word representation.

Examples
```
convertDigitsToPersianWord(0);     // "صفر"
convertDigitsToPersianWord(7);     // "هفت"
convertDigitsToPersianWord(19);    // "نوزده"
convertDigitsToPersianWord(25);    // "بیست و پنج"
convertDigitsToPersianWord(105);   // "یکصد و پنج"
convertDigitsToPersianWord(1234);  // "یک هزار و دویست و سی و چهار"
convertDigitsToPersianWord(-42);   // "منفی چهل و دو"
```
Parameters
- num: number
  The number to convert. Can be positive, negative, or zero.
Returns string
The Persian words for the given number.
Remarks
- Supports numbers up to the billions (میلیارد) as defined in scales.
- Handles special cases for numbers between 10–19.
- Negative numbers are prefixed with "منفی ".
- Defined in functions/convert-digits-to-persian-number/index.ts:70