Avoid eager LazyChoices loading during default validation

[ReinerBRO]

Summary

• avoid eager LazyChoices loading when validating the action's own default
• this prevents Python 3.14 default-validation paths from calling getter() during parser setup
• add a regression test covering default-value validation behavior

Motivation

Fixes #1641.

Testing

• python -m pytest tests/test_cli_utils.py -q

[ReinerBRO]

I dug into the failing CI, and these failures do not appear to be introduced by this PR.

I reproduced the same three failures locally on both:

• this PR branch (3607d94)
• the base commit from master (5b604c3)

The failures are:

• tests/test_cli_ui.py::test_naked_invocation[...] because the argparse invalid-choice wording is currently emitted without quotes around choices
• tests/test_encoding.py::test_terminal_output_response_charset_detection[...]
• tests/test_encoding.py::test_terminal_output_request_charset_detection[...]

For the encoding failures, the current environment resolves the Big5 sample via charset_normalizer as johab, which produces the same mojibake on both commits. So at least from local reproduction, the PR-specific LazyChoices change is not the cause of the red CI.