[fieldset][field] Investigate `Fieldset`s as validation boundaries

# Feature request

## Summary

Mentioned here https://github.com/mui/base-ui/issues/3807#issuecomment-3799942666

`` is currently responsible for creating a validation boundary.

However, it might be more conceptually intuitive to ensure `` only labels one single focusable control, instead of a groups of (identical) controls.

That is:
```jsx
{/* validation boundary for a radio group */}

{/* each radio goes in here /*}

```

Where `Fieldset.Root` can be a validation boundary.

The main issue here seems to be that validation boundaries aren't valid for groups of controls that aren't identical (like a Checkbox Group or especially a Radio Group).

Currently for Checkbox Group and Radio Group, the validation boundary needs to be created by `` and labels need to be placed inside ``:

```jsx
{/* validation boundary for a radio group */}

{/* each radio goes in here for labeling /*}

```