docs(tests): second whole-repo pass over tests (#1103)

[JarryShaw]

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• Followed the coding style (make pylint, make mypy, make isort)

• make test passes, and a test case covers the change. Narrow runs only, listed below.

• Added a changelog entry under docs/source/changelog/ and regenerated CHANGELOG.md, if the change is user-visible: N/A — centralised in docs(changelog): shared 1.5.0 changelog — long-lived, merges last (#610, #616, #617, #618, #620) #657

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Summary

#1103 second whole-repo pass, tests slice.

Prose only: with every string expression statement blanked, each changed
module's AST is identical to origin/main's.

• RFC 959: the case-folding rule sits in Section 5.3, not 4.1, in the
  fix(const): Method.get is case-insensitive, but RFC 9110 makes the method token case-sensitive #896 and test(const): audit every registry case sensitivity against its source specification #903 modules (RFC text, line 2559).
• Counts: the release-gate mutation table's test(project): evaluate the release gates' if: expressions (#962) #966 block is six rows, not
  five; the comment now sums to the table's 21.
• pack(): the HTTP.make calls the versioned make unbound on the class, so it raises TypeError for every real call #452 HTTP docstring describes pack() as it is after
  fix(protocols)!: keep pack() from rewriting the instance it is called on (#1769) #1791, building on _packing_target(); the MPTCP stale-flags case says
  make, which it calls.
• Line references re-measured against origin/main: 23 pointers in 14
  files, into protocol.py, frame.py, field.py, byteorder.py, the five
  UnsupportedCall sites, tcp.py, ipv6.py, extension_header.py, the
  pcapng schema, apptype.py and vendor/main.py.

Refs #1103

[JarryShaw]

Verdict: GOOD TO GO at 79ac0ed73. A Sonnet cross-review (author: Opus) found no blocking findings. Scope stays inside the slice, the AST guard is identical against main, and every directive was checked against the release tags. The merged build of all nine second-pass heads has 18 warnings, the same as main. tests/project: 950 passed, 1 skipped, 3 warnings, 3670 subtests passed.