fix(toolkit): pypcapfile's _default_extension_header still reports a raising trial's warnings #2526

Owner ruling on #2519 (Q2, recommendation taken): the #2308 rule applies here. pcapkit/toolkit/pypcapfile.py's _default_extension_header calls _parse_next_layer and catches a raising trial "as beholder does", but never sets _beholding, so the trial's held warnings replay. Reproduced by #2519's review: HIP b0ff89ff0042ff1c reports packet length < 0 (-16, -32), then returns None.

Fix: set _beholding around that trial, and add a test that fails without it. This also carries #2519's three non-blocking review nits: the half of the docstring about holding outside beholder has no test, the Note's "it" is ambiguous, and the tests use timed words.