test: skip unparseable .py files in legacy-chat regression guard
The two regression tests walked every `*.py` under ROOT (excluding .venv
and __pycache__) and unconditionally ast.parse'd each one. A stray
untracked scratch script in the repo root (filter_vconic_activity.py,
from 2026-04-09) had a syntax error and took both tests down.
Skip files that fail to parse — they can't contain a live Python
import/name/attribute by definition, so the regression guard isn't
weakened by treating them as "not real code."
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>