show help text with invalid default

[davidism]

If ctx.resilient_parsing is enabled, type casting can fail when getting the default, and the raw invalid value will be returned instead. Showing help text temporarily enables resilient parsing mode when fetching the default. I wasn't sure what the effect would be of enabling that for the entire help formatting, so it's just local to the default right now.

• fixes Arguments may break --help #1899

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.