Avoid trailing comma for single-argument with positional separator (#9076)
## Summary
In https://github.com/astral-sh/ruff/pull/8921, we changed our parameter
formatting behavior to add a trailing comma whenever a single-argument
function breaks. This introduced a deviation in the case that a function
contains a single argument, but _also_ includes a positional-only or
keyword-only separator.
Closes https://github.com/astral-sh/ruff/issues/9074.