Support multi-select shadow

Problem/Motivation

The Box Shadow style plugin was updated to support multi-select (checkboxes) so multiple
shadow classes can be applied at the same time. However, the automatically injected
“N/A” option (_none) causes incorrect behavior when used with checkboxes.

“N/A” can be selected together with other shadow options, even though its purpose is
to represent no shadow at all.

Steps to reproduce

1. Enable the Box Shadow style plugin.
2. Open Layout Builder and edit a section or block.
3. Select multiple Box Shadow options.
4. Select the “N/A” option.
5. Observe that “N/A” remains selected alongside other shadow options.

Proposed resolution

Update the Box Shadow style plugin so the “N/A” option behaves as a reset:

• Selecting “N/A” unselects all other shadow options.
• Selecting any shadow option unselects “N/A”.
• “N/A” is never stored or applied as a CSS class.

Remaining tasks

• Review and agree on expected behavior for “N/A” in multi-select shadow styles.
• Add or update tests if required.

User interface changes

Improves the Box Shadow selection UI by preventing invalid combinations and making
“N/A” behave consistently as a reset option.

API changes

None.

Data model changes

None.