fix: use extension registry for CLI text parsing

[wackywendell]

Description

The CLI's convert -f text and validate commands didn't pass the ExtensionRegistry to the text parser, so plans containing custom extensions (registered via run_with_extensions) failed to parse from text input.

This PR:

• Adds parse_with_registry() to lib.rs as the symmetric counterpart to format_with_registry()
• Fixes Format::read_plan for text input to use the registry-aware parser
• Refactors run_validate_with_io to reuse Format::read_plan / write_plan instead of duplicating the parse/format logic

Type of Change

• Bug fix

Testing

• All existing tests pass (176 unit + 85 integration/doc tests)
• Pre-commit hooks pass (clippy, fmt)

Checklist

• Code follows Rust conventions
• Documentation updated if needed
• No breaking changes (or breaking changes documented)
• Pre-commit hooks pass

Related Issues

Closes #95

[benbellick]

Looks reasonable to me. Though I wonder if there is a way to test this. Maybe we could have some dummy protos inside of this repo for testing purposes?