Add clear() method to RadioSet

[nikhil2004-blip]

Description

Adds a clear() method to RadioSet that deselects all radio buttons, returning the set to its initial unset state.

Changes

• Implements RadioSet.clear() to deselect all buttons
• Returns RadioSet to initial unset state
• Updated RadioSet._on_radio_set_changed() to handle cleared state (index=-1)
• Updated RadioSet.Changed.__init__() to accept pressed: RadioButton | None
• Useful for surveys/questionnaires where no response is valid
• Fixes Adding an option to clear the RadioSet #6198

Motivation

As discussed in #6197, this enables use cases where an unset RadioSet is needed, such as survey interfaces where "no response" is a valid state, without requiring a "Skip" button workaround.

Usage Example

from textual.widgets import RadioSet

radio_set = RadioSet("Yes", "No")
# User selects something...
radio_set.clear()  # Now nothing is selected

Testing

Added comprehensive tests in tests/test_radio_set_clear.py covering:

• Basic clear functionality
• Clearing empty RadioSet
• Multiple clears (idempotent)
• Selecting after clearing

Please review the following checklist.

• [✅ ] Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

after review i can update documentation and CHANGELOG.md