fix(dpkt): extract(engine='dpkt') lets a raw AttributeError escape on an IPv6 Fragment header followed by ESP or Destination Options #1351

Describe the bug
dpkt 1.9.8 dpkt/ip6.py:96 evaluates ext.frag_off for any header whose predecessor has nxt == 44, so an IPv6 Fragment header followed by ESP (50) or Destination Options (60) raises AttributeError: 'IP6ESPHeader' object has no attribute 'frag_off'. That is upstream, but pcapkit/foundation/engines/dpkt.py:163 (packet = protocol(pkt)) does not catch it, so the raw error aborts the whole extraction instead of an in-library error.

To Reproduce
One Ethernet frame, IPv6 nxt=44, Fragment header 32 00 00 00 00 00 00 01 (next header ESP, offset 0, M=0) + 8 octets, in a pcap:
pcapkit.extract(fin='x.pcap', engine='dpkt', nofile=True, store=False) -> AttributeError: 'IP6ESPHeader' object has no attribute 'frag_off'. Next header 60 (payload 3b00010400000000) raises 'IP6DstOptsHeader' ... 'frag_off'.

Expected behavior
The engine converts a dpkt parse failure into a pcapkit error (or skips the frame with a warning) rather than leaking the third-party exception.

Reproduced on main at c9676c5 (check D5).

Found by the PR cross-review follow-up round-trip audit (#1202). The repro was re-checked by an independent verifier, and I re-ran it on main at c9676c563.