Support Python 3.14 unparenthesized exception lists (PEP 758)

[PEP 758](https://peps.python.org/pep-0758/) says this is legal in Python 3.14:

```py
try:
x = 1 / 0
except ZeroDivisionError, ValueError, IndexError:
pass
```

but YAPF chokes on this.

Start by reproducing the failure with the PEP 758 example in YAPF, then trace the parser or formatting entry point that rejects the unparenthesized exception list. The work is complete when valid Python 3.14 syntax is accepted and formatted without regressing existing exception-handling cases.

Written by the indexing model from the issue text.