chore: convert remaining /next components from BEM to flat class names

[millus]

Several `/next` components were built before the flat class name convention was established and still use BEM (`__` element separators, `--` modifier suffixes). These should be updated to use simple flat class names scoped by CSS nesting, as documented in AGENTS.md.

Components to update

• Radio
• Checkbox
• Input
• Field
• Switch
• TextField
• Banner

Breaking change

Renaming CSS classes is a breaking change for any consumer who is targeting these class names directly in their own CSS (e.g. `.eds-field__helper-message { color: red }`). Each fix should be committed as `fix!:` and the class name changes documented in the PR description so consumers know what to update.

Testing checklist (per component PR)

• Update any snapshot tests that reference the old class names
• Verify the component visually in Storybook matches the Figma design
• Confirm `jest-axe` still passes after the rename

Related

ADR 0006 documenting the flat class name convention and superseding the BEM-inspired naming in ADR 0002 is in #5134.

[claude]

This may be related to a previous issue:

• docs: update CSS convention from BEM to CUBE CSS-inspired nesting #4748 — docs: update CSS convention from BEM to CUBE CSS-inspired nesting (closed): established the CUBE CSS flat-naming convention that this issue rolls out to the remaining components; closed as completed, so it documents the target convention these components should be converted to.

This comment was generated by an LLM and may be inaccurate. Please verify that the linked issues are actually relevant.