[WC-3474] fix check box validation feedback width

[rahmanunver]

Summary

• Check Box validation feedback was not consistently full width — its height depended on the label's length, unlike other inputs.
• Root cause: .mx-checkbox.label-after is a flex container without flex-wrap, so .mx-validation-message's flex-basis: 100% couldn't force it onto its own row.
• Added flex-wrap: wrap to .mx-checkbox.label-after in _check-box.scss.

Test plan

• Verified in browser: validation message now renders full width with fixed height, independent of label length
• Verified no-error state unaffected (no stray wrapping)
• Confirmed no other widget shares this scss block (no regression surface)