feat(button): use DS background token for contrast

[tamas-sage]

Proposed behaviour

In Button/Test -> AllSizesVariantsTypes, some button groups (including secondary destructive examples) are rendered on hardcoded grey backgrounds (#DDDDDD). This can produce contrast failures and does not represent an official DS background token.

Current behaviour

Use Design System background tokens in the story (e.g. var(--mode-color-generic-bg-nought)) instead of hardcoded greys, so contrast checks run against approved DS colors and reflect intended design usage.

Checklist

• Commits follow our style guide
• Related issues linked in commit messages if required
• Screenshots are included in the PR if useful
• All themes are supported if required
• Unit tests added or updated if required
• Playwright automation tests added or updated if required
• Storybook added or updated if required
• Translations added or updated (including creating or amending translation keys table in storybook) if required
• Typescript d.ts file added or updated if required
• Related docs have been updated if required

QA

• Tested in provided StackBlitz sandbox/Storybook
• Add new Playwright test coverage if required
• Carbon implementation matches Design System/designs
• UI Tests GitHub check reviewed if required

Additional context

Testing instructions

[edleeks87]

@tamas-sage apologies I've just realised this has a feat commit which will trigger a release which we don't need for this change. Either chore/docs/refactor will be better for this