Fix LazyChoices for Python 3.14+ — avoid eager getter call during parser registration

[driphtyio]

Problem

On Python 3.14+, argparse validates that default is in choices at argument registration time (in add_argument()). The LazyChoices class was setting self.choices = self, which caused argparse to call __contains__ → load() → getter() during parser construction — triggering the getter prematurely.

This broke the invariant in test_lazy_choices_help that the getter must not be called during parser initialization.

Fixes #1641.

Solution

Two simple changes:

1. Removed self.choices = self — without it, action.choices stays None (argparse default), so Python 3.14's registration-time check is skipped entirely.

2. Moved validation into __call__ — when a user-provided value reaches the action handler, we call self.load() (which triggers the getter) and raise ArgumentError if the value isn't valid.

The lazy-loading contract is preserved: the getter is only called when a value is actually parsed or help is displayed.

Testing

• test_lazy_choices — passes (invalid choice still raises SystemExit)
• test_lazy_choices_help — passes (getter not called during init)
• test_parser_serialization — passes
• Full test suite: 260 passed, 1 pre-existing failure (unrelated encoding test)