fix(loader): inherit text colour from parent Button, if it exists

[Parsium]

closes #7638

Proposed behaviour

If Loader is rendered within a Button, use its text colour to style the label and inner ring arc. This ensures sufficient colour contrast between the label text and Button background, even when hovered:

Screen.Recording.2025-11-24.at.12.09.04.mov

Current behaviour

When Loader is rendered within a secondary or tertiary Button, there is insufficient colour contrast between the label text and button background:

Screen.Recording.2025-11-24.at.12.08.07.mov

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

This is a temporary fix while #7624 is under development. The designs of the updated Button include new hover colours for secondary and tertiary buttons, which shouldn't have this contrast issue.

Testing instructions

[carbonci]

🎉 This PR is included in version 158.13.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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