[radio group] disabled radio group recieves focus

# Bug report

## Current behavior

The native HTML radio when disabled, does not receive focus on keyboard tab, or programatic focus. The Base UI Radio Group will gladly take focus even though it's disabled.

## Expected behavior

`RadioGroup` and `Radio` when disabled should not receive focus.

## Reproducible example

Open Blitz and keyboard tab. Programatic focus also works
https://stackblitz.com/edit/bf5bx4pd-gxtnshkq?file=src%2FApp.tsx

## Base UI version

For example v1.2.0

## Which browser are you using?

Edge

## Which OS are you using?

Mac OS

## Additional context

I believe the issue is due to `tabIndex=0` being set on the Radio, even though it's `disabled`.

Im trying to build a "focus first field" on a form when it's mounted, and when a RadioGroup is used as the first element, `tabbable` (the library Im using to determine focus) is seeing the RadioGroup as a valid target.

There's also a strange mounting behavior going on with RadioGroup, it first mounts with `tabIndex=-1` for RadioGroups that are not disabled, then after a render they appear with `tabIndex=0`.