feat(picker): add optional interactive search/filter support

[MRThugh]

Summary

This PR introduces an optional interactive search/filter feature to the Picker UI component and the top-level pick() function.
Users can now type to dynamically filter the options list using case-insensitive substring matching.

This enhancement significantly improves usability for long option lists, while maintaining full backward compatibility.

Details

• Added a new parameter: enable_search: bool = False to both Picker and pick() function.
• When enabled, the UI shows a "Search: ..." prompt where user keystrokes update the filtered list in real-time.
• If the search yields no matches, a “No results” message is displayed instead of empty options.
• Internal state management updated to ensure index safety, preventing IndexError when filtering modifies the list.
• Navigation with arrow keys respects the filtered subset.
• Works transparently with both backend implementations (curses and blessed).
• Default behavior remains unchanged for users who do not enable this feature, ensuring backward compatibility.
• Code style and formatting follow existing project conventions.

Testing

• Manual testing performed on Python 3.10+ in terminal environments.
• Verified navigation, selection, and search input (including backspace).
• Tested with both curses and blessed backends.

Impact

• Non-breaking feature addition.
• Improves experience for end-users dealing with large lists.
• Lay groundwork for future enhancements like fuzzy search (optional).