Add data-style attributes for resolved style fields

[sneridagh]

Summary

• preserve resolved style field values in @plone/helpers even when the selected definition has no style object
• expose resolved style field values from @plone/plate as data-style-* attributes on editor element props
• add towncrier news entries for both affected release targets

Changes

• packages/helpers/src/styleFields.ts now records the effective semantic value before checking whether the matching definition contributes CSS, so consumers can still access the resolved value when a style field is value-only
• packages/helpers/src/styleFields.test.ts adds coverage for definitions without a style object to lock in the returned { values, style } contract
• packages/plate/components/editor/plugins/style-fields-plugin.ts now converts resolved style-field values into data-style-* element props and keeps merging CSS styles when present
• packages/plate/components/editor/plugins/block-width-plugin.test.ts adds assertions for the new data-style-blockWidth and data-style-theme attributes so the Plate output contract is explicit
• packages/helpers/news/+style-field-values.bugfix documents the helpers fix
• packages/plate/news/+style-field-data-attrs.feature documents the new Plate capability

Testing

• pnpm --filter @plone/helpers test --run src/styleFields.test.ts
• pnpm --filter @plone/plate test --run components/editor/plugins/block-width-plugin.test.ts
• pnpm --filter @plone/plate check:ts