[ozenyunus35]

Fixes #466

Changes proposed in this pull request:

• intcomma(value, ndigits) with a negative ndigits used to fail with ValueError: Format specifier missing precision from the f-string. It now raises ValueError: ndigits must be >= 0, got -1 before doing anything else, as suggested in the issue, and the docstring lists it under Raises.
• Added test_intcomma_negative_ndigits for float, int, numeric string and None values; it fails on main.

An alternative would be to round to tens/hundreds like round(x, -1), but the docstring describes ndigits as digits after the decimal point, so an error seemed like the smaller change. Happy to switch if you'd prefer that.

I used an AI assistant to help draft parts of the code and tests; I reviewed the changes and ran the tests myself.

[hugovk]

Duplicate of #467, as you can see linked from #466.