fix(select): avoid invalid radio group structure

[kavyansh18]

Issue number: resolves #31074

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What is the current behavior?

When ion-select uses interface="modal" in single-select mode, the modal renders an ion-list containing an ion-radio-group.

The ion-list has role="list" while the nested ion-radio-group has role="radiogroup", resulting in an invalid ARIA structure for the radio options.

What is the new behavior?

Single-select modal options are now rendered directly inside the ion-radio-group without the wrapping ion-list.

• Removes the conflicting role="list" wrapper from the single-select radio options.
• Preserves the existing ion-list structure for multiple-select mode.
• Preserves the existing MD spacing and radio option styling after removing the list wrapper.
• Adds regression coverage for the single-select and multiple-select DOM structures.

Does this introduce a breaking change?

• Yes
• No

Other information

The change is limited to the select modal rendering, its existing MD styles, and regression tests.

Multiple-select behavior is unchanged.

The relevant select modal tests, linting, Stylelint, Prettier, and build checks pass.

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