Don't format trailing comma for lambda arguments (#5946)
**Summary** lambda arguments don't have parentheses, so they shouldn't
get a magic trailing comma either. This fixes some unstable formatting
**Test Plan** Added a regression test.
89 (from previously 145) instances of unstable formatting remaining.
```
$ cargo run --bin ruff_dev --release -- format-dev --stability-check --error-file formatter-ecosystem-errors.txt --multi-project target/checkouts > formatter-ecosystem-progress.txt
$ rg "Unstable formatting" target/formatter-ecosystem-errors.txt | wc -l
89
```
Closes #5892
