feat(picker): add select_all and deselect_all helper methods

[miskatul-anwar]

Summary

Adds select_all() and deselect_all() helper methods to Picker for multiselect mode:

• Picker.select_all(): Selects all available options that are enabled (Option.enabled is not False).
• Picker.deselect_all(): Clears selected_indexes.
• Both methods safely return immediately if multiselect=False.

Testing

• Added unit test test_select_all_and_deselect_all in tests/test_pick.py.
• Ran full pytest test suite (all passing).
• Ran mypy type checks (clean).