Fix fail-closed handling for pickle parse errors in joblib files

[phenggeler]

When pickletools.genops() encounters unparseable bytes mid-stream (common in joblib hybrid serialization), parsing halts without extracting remaining opcodes. Previously this was silently treated as "no issues found". Now parse failures in pickle-like files trigger a CRITICAL sentinel finding.

Changes:

• Add _source_looks_like_pickle_stream() to detect pickle-like extensions
• Modify _list_globals() to preserve parse context and defer error handling
• Modify scan_pickle_bytes() to fail-closed when parse fails with no extracted globals
• Fix aggregation bug: errors and issues were mutually exclusive due to if/elif logic
• Add unit tests for sentinel behavior