feat(picker): support negative default_index

[miskatul-anwar]

Summary

This PR adds support for negative default_index values (e.g. default_index=-1 to select the last option by default), following standard Python negative indexing conventions.

• Bounds checking ensures -len(options) <= default_index < len(options).
• Negative indices are converted to their corresponding positive index upon initialization.
• Added comprehensive unit tests in tests/test_pick.py.

Testing

• Ran pytest test suite with all tests passing.
• Ran mypy type checks with 0 errors.

[aisk]

Why the try/except around cadule? It's already declared in pyproject.toml, so if it's missing the install is already broken, and this just turns a clear ImportError into a confusing TypeError later on.

Also this PR is doing too many unrelated things at once. Please split it up, one PR per feature.

[miskatul-anwar]

Thanks for the review @aisk!

I've addressed both points:

1. Removed the unnecessary try...except block around cadule.
2. Scoped this PR solely to the negative default_index support feature.

I will open separate PRs for the disabled option safety fix in mark_index() and the select_all() / deselect_all() helper methods.