Fix FieldGroup disappearing on resize inside FieldSet

[7azemaamer]

Summary

Fixes an issue where FieldGroup content disappears on viewport resize when used
inside FieldSet alongside a sibling that switches display at a breakpoint.

Details

Applying container queries directly on <fieldset> can trigger a browser reflow
issue when a sibling element changes layout (e.g. grid → flex), causing the
container query context to break while content remains in the DOM.

This change keeps <legend> as a direct child of <fieldset> (HTML spec
compliant) and wraps remaining children in an inner <div> to isolate layout
and container queries.

Notes

This is a non-breaking change. The only behavioral difference is that selectors
like fieldset > [data-slot="field-group"] no longer match, which is unlikely to
affect real-world usage.

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