check-yaml: report invalid UTF-8 instead of raising a traceback

[0xacee]

Problem

check-yaml opens each file as UTF-8 text, but the parse is only guarded against ruamel.yaml.YAMLError. A file that is not valid UTF-8 (for example latin-1) makes load_fn raise UnicodeDecodeError, which escapes main():

$ printf 'key: caf\xe9\n' > latin1.yaml
$ check-yaml latin1.yaml
Traceback (most recent call last):
  ...
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 8: invalid continuation byte

The exception aborts the loop, so any remaining files are never checked, and the user gets a stack trace instead of a diagnostic.

The sibling hooks already handle this cleanly: check-json catches ValueError (a superclass of UnicodeDecodeError), and check-toml gets a TOMLDecodeError from tomllib.

Change

Catch UnicodeDecodeError next to YAMLError and report filename: invalid UTF-8: ..., so the hook marks the file as failed with a readable message and still checks the rest of the arguments.

Testing

• New regression test test_main_non_utf8_file fails on main (UnicodeDecodeError propagates) and passes with the fix.
• pytest tests/ — 414 passed, 4 xfailed.

[asottile]

we don't accept ai slop