Fix convert help subcommand error for #210

[heathdutton]

Committer Notes

Closes #210.

The convert subcommand was throwing an error when invoked with --help, instead of displaying the help message and returning exit code 0. This occurred because the parent class marked the --to option as required, causing parsing to fail before help could be displayed.

This fix overrides gatherOptions() to define the --to option without marking it as required, and implements validateOptions() to skip validation when --help or --version is present. For normal convert operations, --to is still validated manually to ensure it's provided.

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[iMichaela]

Thank you, @heathdutton. @selenaxiao-nist - please review the PR when you have a chance.

[selenaxiao-nist]

@heathdutton
Thanks for your contribution. This removes the error when running oscal-cli catalog convert --help but prints the error twice when running oscal-cli catalog convert

Please revise.

[selenaxiao-nist]

The error is still printed twice, but after more testing I found this also happens with other subcommands, and isn't directly caused by these new changes. LGTM otherwise. I'll approve this PR and open a new issue.