fix(probes): guard badchars ASCII selection against max_ascii_variants=1

[manunicholasjacob]

What this changes

BadCharacters._select_ascii() divides by limit - 1:

if limit is None or limit <= 0 or limit >= len(ASCII_PRINTABLE):
    return list(ASCII_PRINTABLE)
step = max(1, (len(ASCII_PRINTABLE) - 1) // (limit - 1))

limit == 1 passes both guards, so the division raises ZeroDivisionError.
max_ascii_variants is configurable via DEFAULT_PARAMS, so setting it to 1
prevents the probe from constructing.

_select_positions() in the same class already handles the equivalent case:

if cap == 1:
    return [positions[0]]

This adds the matching guard to _select_ascii().

Reproduction

probes:
  badchars:
    BadCharacters:
      max_ascii_variants: 1

fails at probe construction with integer division or modulo by zero. Values
of 2 and above work as expected.

Testing

Adds tests/probes/test_probes_badchars.py covering limits 1, 2 and 3. The
limit-1 case fails on main and passes with this change.