Align ARGS_NOEXCEPT completion behavior with throwing mode

[jmestwa-coder]

Summary

This fixes an inconsistency between throwing mode and ARGS_NOEXCEPT during shell completion parsing.

In throwing mode, recursive parsing of partial completion input suppresses ordinary parse failures and returns empty completion output. ARGS_NOEXCEPT did not mirror that behavior and could leak Error::Parse state for unknown commands during completion.

Changes

• Restore Error::Completion state after recursive completion parsing in ARGS_NOEXCEPT
• Clear leaked parse error messages from inner completion parsing
• Add focused regression coverage for unknown-command completion in noexcept mode

Result

ARGS_NOEXCEPT completion behavior now matches the existing throwing-mode semantics for partial/unknown command completion input.

[Taywee]

Makes sense to me. The nothrow code has had a lot of issues being consistent with the throw code. It's a fairly half-baked feature.

Thanks for the PR.