fix(sctp): accept a final parameter or cause short of its padding (#1479)

[JarryShaw]

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nested_length sizes a chunk's parameter or cause list to include the chunk's trailing padding. When the last chunk omits that padding (RFC 9260 §3.2), the last item read past the data and raised ProtocolError. The six nested lists (INIT, INIT ACK, HEARTBEAT, HEARTBEAT ACK, ABORT, ERROR) now wrap the callback in bounded, so the span stops where the data ends. The #1477 short-padding record already rebuilds the item byte-exactly.

ASCONF has no dedicated schema. It parses as an opaque chunk and was not affected.

The issue's INIT repro now parses. Its host name abcde has no NUL terminator, so _make_param_hostname still refuses to rebuild it. That check is older and fails the same way when the parameter is padded, so it is left for a separate issue. The new tests use abcd\0.

Tests: the new test_sctp_unpadded_final_param_unit.py fails 36/36 on main and passes on this branch. test_sctp*.py, test_transport_edge_roundtrip_unit.py and tests/project pass. repr(info.to_dict()) hashes over examples/captures (23 files) are unchanged.

Closes #1479

[JarryShaw]

Verdict: GOOD TO GO at 3d721832a. Cross-reviewed on Sonnet; the author ran on Opus.

• The fix: all 6 nested_length( call sites in schema/transport/sctp.py are now bounded(...); I counted them myself. On origin/main sources the new tests give 36 failed, 4 passed; at the head, 5 passed, 35 subtests passed.
• No input that should raise now passes. The reviewer cut five INIT, INIT ACK, ABORT, ERROR and HEARTBEAT packets at every offset, 271 cuts in all. Only the 4 full-length unpadded packets change result: ProtocolError on main, parsed byte-exact at head. 56 malformed packets with overruns, oversized chunk lengths and short fixed headers behave the same on both trees.
• No other list was missed. The other ListFields were already bounded. ASCONF and ASCONF-ACK parse as unknown chunks and round-trip at head.
• Regression checks: the per-capture to_dict() hashes are identical across all 23 captures. The SCTP test files give 72 passed, 292 subtests.

The Host Name rebuild failure behind #1479's literal repro predates this PR and is tracked in #1482.

[github-actions]

Coverage: 89.57% (unit tier, Python 3.14, 3d721832a, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	4	20	4	93.55%
pcapkit/const	18795	958	2356	871	90.99%
pcapkit/corekit	2107	93	672	27	94.60%
pcapkit/dumpkit	242	2	88	2	98.79%
pcapkit/foundation	2648	98	930	48	94.80%
pcapkit/interface	112	7	40	5	92.11%
pcapkit/protocols	17086	239	4628	192	97.97%
pcapkit/toolkit	539	74	168	3	84.58%
pcapkit/utilities	431	4	124	4	98.56%
pcapkit/vendor	4409	2342	1006	157	43.25%

Per-file detail: the coverage-html artifact of this run.