fix(const): make Command.get reject a non-str key as Method.get does (#1824)

[JarryShaw]

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---

Description

Closes #1824. Command.get was a staticmethod that called key.upper() on any key:

• get(b'\x01') minted a member keyed by bytes;
• get(-1) raised a builtin AttributeError and ignored default.

It now follows Method.get (#908). It is a classmethod, and a non-str key goes to EnumLookup.get, whose _validate_value (from #1823) raises EnumValueError. Mirroring Method.get, default does not rescue a refused key: this override's default is the value of the unregistered member it builds, not a registered fallback.

A str key behaves exactly as before: a case-insensitive lookup, with the unregistered-member fallback in the caller's casing.

• Template: changed in pcapkit/vendor/ftp/command.py, and the const file was regenerated offline. An AST check against main shows that only Command.get and the _validate_value docstring changed, and no member data. FEATCode.get was already correct.
• Directive: .. versionchanged:: 1.5.0 on Command.get. registry-protocol.rst and test_conventions_doc_claims.py no longer list it as a staticmethod.
• Tests: the new tests/const/test_const_ftp_command_get_key_type_1824_unit.py covers each key type, a key paired with a default, case handling and parity with Method.get. It fails 9 times on main. tests/const (355 passed) and tests/project (959 passed, 1 skipped) pass.

[JarryShaw]

Verdict: GOOD TO GO at 9409c4589. A Sonnet cross-review (author: Opus) re-derived the following:

• Non-str keys: bytes, int, None, float and container keys all raise EnumValueError, including when a default is given, which matches Method.get on main.
• str keys: behaviour is identical to main, row for row.
• Callers: the library's only caller passes a decoded str.
• Generated file: the template, rendered against live IANA data, is byte-identical to the committed const file. The AST diff touches only Command.get and the _validate_value docstring.
• Mutations: eight were applied, including restoring staticmethod, forwarding default, accepting bytes and case-sensitive lookup. The new tests caught every one.
• Test suites: tests/const, tests/vendor and tests/project pass.

[github-actions]

Coverage: 90.05% (unit tier, Python 3.15, 9409c4589, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	2	20	2	96.77%
pcapkit/const	19595	881	2618	867	92.11%
pcapkit/corekit	2462	69	780	35	96.55%
pcapkit/dumpkit	276	3	100	3	98.40%
pcapkit/foundation	2915	164	1016	34	93.59%
pcapkit/interface	138	7	52	5	93.68%
pcapkit/protocols	17408	224	4580	189	98.10%
pcapkit/toolkit	872	177	298	8	77.18%
pcapkit/utilities	512	2	158	4	99.10%
pcapkit/vendor	4407	2342	1010	157	43.27%

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