nsh: dissect MD-Type 2 variable length context headers

[mmadersbacher]

Any NSH MD-Type 2 packet carrying context headers fails to dissect:

>>> NSH(hex_bytes('0fc40203000001ff00010204deadbeef'))
TypeError: 'str' object is not callable

count_from was given the string 'length' instead of a callable, in NSH.vlch and in NSHTLV.metadata.

count_from is also the wrong parameter here. RFC 8300 section 2.2 defines the base header length as "the total length, in 4-byte words, of the NSH", not a count of context headers, and section 2.5.1 defines the TLV length as the metadata length in bytes, padded up to a word boundary. Both become length_from, and the base header length is computed with length_of instead of count_of. NSHTLV also needs extract_padding, otherwise the first TLV swallows the ones behind it.

After the change:

>>> p = NSH(hex_bytes('0fc60203000001ff00010204deadbeef00020104cafebabe'))
>>> len(p.vlch), p.vlch[1].class_, p.vlch[1].metadata
(2, 2, [3405691582])

MD-Type 1 is unaffected. The existing tests stay green because they only exercise the build path, and the one MD-Type 2 case among them carries no context headers.

Validated on Windows with Python 3.13. UTscapy -t test/contrib/nsh.uts -P "load_contrib('nsh')" gives 9 passed, 0 failed, with three cases added: one TLV, two TLVs, and a build check on the computed length. TLV lengths that are not a multiple of 4 dissect correctly when padded as the RFC requires. Malformed base header lengths and truncated context headers do not raise; truncated TLV metadata raises struct.error, same as other truncated layers in scapy. flake8 scapy/ and mypy_check.py (win32 and linux) are clean. The full windows.utsc campaign gives 96 failures, the same count and the same files as on master, all unrelated and caused by missing tcpdump, tshark and libpcap here.

The test vectors are hand-built from RFC 8300, not taken from a capture.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.58%. Comparing base (1f87020) to head (be7c3c2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5096      +/-   ##
==========================================
+ Coverage   79.42%   80.58%   +1.15%     
==========================================
  Files         372      390      +18     
  Lines       96507    96860     +353     
==========================================
+ Hits        76652    78053    +1401     
+ Misses      19855    18807    -1048     

Files with missing lines	Coverage Δ
scapy/contrib/nsh.py	96.15% <100.00%> (+0.32%)	⬆️

... and 304 files with indirect coverage changes

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