fix(foundation): the pypcap engine raises a pcap loader's OSError instead of falling back as pcap_ct does #2132

pcapkit.extract(engine='pypcap') raises the loader's OSError when a pcap module is installed but cannot load. That happens, for example, with pcap-ct on macOS (dlopen(libc.so.6 …)), on Windows without Npcap (Cannot find npcap/wpcap.dll), or on Linux without libpcap.so. The same situation with engine='pcap_ct' warns and falls back to the default engine.

Cause: PyPCAP._reason_for in pcapkit/foundation/engines/pypcap.py only returns _pcap_backend.wrong_backend_reason(...). PCAP_CT._reason_for also reports a probe whose module is installed but unusable (found.name is None and not found.missing), with the OSError as the reason.

Reproduced in the #2131 review with a fake pcap package whose import raises OSError('Cannot find libpcap.so library'), put on sys.path in a subprocess:

• engine='pcap_ct' warns and falls back.
• engine='pypcap' raises OSError.

Found while root-causing #2129, whose benchmark-side workaround is #2131.

Fix: give PyPCAP._reason_for the same installed-but-unusable branch, or move that branch into a helper in _pcap_backend that both engines call. Add a test with a fake pcap package that fails without the fix.