fix(const): Command.get calls key.upper() on any key, so bytes mint and ints raise AttributeError #1824

Describe the bug
Command.get (pcapkit/const/ftp/command.py:551, generated from pcapkit/vendor/ftp/command.py) is a staticmethod that calls key.upper() without checking the key's type. Measured on main:

• Command.get(b'\x01') returns <Command.b'\x01': None>, a bogus unregistered member keyed by bytes.
• Command.get(-1) raises AttributeError: 'int' object has no attribute 'upper', which is a builtin error rather than an in-library one, and default is ignored.

Method.get had the same shape. #908 fixed it: it became a classmethod that delegates to EnumLookup.get and rejects a non-str key with ValueError (see its versionchanged:: 1.5.0).

Expected behavior
Command.get follows Method.get: a non-str key is rejected with the in-library error, and a str key keeps today's case-insensitive lookup and unregistered-member fallback. A test pins each case.

Additional context
Found by #1816's worker (#1823). Blocked until #1823 merges, because both change pcapkit/vendor/ftp/command.py and the const file it generates.