test(tcp): retarget the last assertion pinning the old padding side (#604)

[JarryShaw]

Follow-up to #621, which fixed #604. main is red without this.

What is broken on main

TCPUDPUnitTests.test_a_truncated_option_still_parses_its_declared_length
(tests/protocols/transport/test_tcp_udp_unit.py:1415 on main) pins the padding
side that #604 corrected:

self.assertEqual(unassigned.data, b'\x00' * zeroes + trailing)

A truncated TCP option's data is the octets that were actually read followed by
the zeros synthesised for the ones that were not — rjust() produced the reverse,
and #621 changed FieldBase.unpack to ljust(). Both parametrised widths
(declared_length=12 and 32) therefore fail.

Measured against main at 2221c2d8f, from a git archive export with
pcapkit.__file__ asserted inside the export before any other import:

# main as it stands
2 failed, 1 passed, 16 deselected, 4 warnings in 1.43s
  SUBFAILED(declared_length=12) ...::test_a_truncated_option_still_parses_its_declared_length
  SUBFAILED(declared_length=32) ...::test_a_truncated_option_still_parses_its_declared_length

# with this change
1 passed, 16 deselected, 3 warnings, 2 subtests passed in 0.64s

Why it was not in #621

#621 deliberately left it. This file was concurrently owned by #612 at the time,
and editing it would have risked discarding work that has since merged — #621's
own body called that out and named the exact one-line change needed. #612 merged,
and then #621 merged before the follow-up could be folded into it, so the
correction lands here instead.

The change

• The assertion becomes trailing + b'\x00' * zeroes, for both widths.
• The docstring above it said FieldBase.unpack "left-pads the short read … a
  data value of four zero octets followed by the six real ones". It now says
  tail-pads, and the six real octets followed by four zero ones, with a note that
  before FieldBase.unpack pads a short read with rjust regardless of byte order, silently corrupting little-endian values #604 it was the other way round.
• A changelog bullet, and CHANGELOG.md regenerated with util/changelog_md.py
  (--check exits 0).

Test-only — no library code changes. The sibling case in
tests/protocols/internet/test_ipv4_unit.py was already retargeted in #621, and
bytes(proto.__header__) == raw on the line below the changed assertion passes
untouched, since the pack path is byte-preserving and is not affected by the
padding side.

Verification

tests/protocols/transport/ + tests/corekit/test_fields_field.py
    177 passed, 232 subtests passed, 0 failed

Run with PYTHONSAFEPATH=1 and PYTHONPATH set to the tree under test, with
pcapkit.__file__ and the padding side both asserted before the run — this
machine's venv editable install otherwise resolves pcapkit to a checkout that is
behind origin/main, which silently invalidates a bare run.