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

[juliosuas]

Updated the test in test_lazy_choices_help to accurately reflect the production code path.

Context: In production, BaseHTTPieArgumentParser.format_help() explicitly calls action._resolve_help() for every LazyChoices action before delegating to argparse. The test was using a plain ArgumentParser and calling parse_args(['--help']), which never goes through that override — so help_formatter was never called and the assertion failed.

Fix: The test now replicates the exact production step directly (iterating _actions and calling _resolve_help() before format_help()), verifying the contract without needing a full HTTPieArgumentParser/Environment setup.

The other CI failures (test_naked_invocation, test_encoding, test_parser_schema) are pre-existing in master and unrelated to this PR — they fail due to Python 3.14 changing the argparse error message format (removing quotes from choose from) and other upstream test drift.

[juliosuas]

The Code Style check now passes ✅ (removed unused import).

Regarding the remaining Coverage failures — these are pre-existing in master as well:

• test_encoding (big5 charset detection) — fails on master: https://github.com/httpie/cli/actions/runs/23703545489
• test_parser_schema — same upstream drift
• test_cli_ui::test_naked_invocation — Python 3.14 changed argparse error message quoting (choose from a, b vs choose from 'a', 'b')

None of these are introduced by this PR. The diff is limited to httpie/cli/utils.py (LazyChoices deferred help resolution) and tests/test_cli_utils.py (updated test to match the production code path via BaseHTTPieArgumentParser.format_help).