fix: defer LazyChoices getter call until --help is used (Python 3.14 compat)

[juliosuas]

Summary

Closes #1641.

Python 3.14 added a _check_help() call inside ArgumentParser.add_argument() that reads action.help during parser construction. Previously, only parse_args(['--help']) triggered help formatting.

LazyChoices was designed to call getter() lazily — only when --help is passed — but Python 3.14 caused getter() to be invoked eagerly during add_argument(), breaking test_lazy_choices_help.

Root Cause

LazyChoices.help was a property that called self.load() (which calls getter()) whenever the help string was requested. In Python 3.14, argparse accesses action.help inside add_argument() via _check_help() → _expand_help() → _get_help_string() → action.help.

Fix

Two small changes:

1. httpie/cli/utils.py — Separate resolution from the property:

• Add _resolve_help() method that calls getter() and formats the help (deferred)
• Make the help property return self._help as-is without triggering loading
• Add _help_resolved flag to track state

2. httpie/cli/argparser.py — Override format_help() in BaseHTTPieArgumentParser:

• Before rendering help, call action._resolve_help() on all LazyChoices actions
• This ensures getter() is called exactly once and only when --help is actually used

Verification

All test_lazy_choices_help assertions pass on Python 3.14:

• ✓ getter not called during add_argument()
• ✓ getter / help_formatter not called after parse_args([])
• ✓ help_formatter not called when passing a value without --help