fix: textarea "responsiveness" due to CSS selector

[mfranzke]

Proposed changes

As reported in issue #5561, some of our selectors seem to cause a laggy UI, especially with textarea elements. Therefore, we should optimize them and measure the results.

resolves #5561

Types of changes

• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improvements to existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

🔭🐙🐈 Test this branch here: https://design-system.deutschebahn.com/core-web/review/5561-v421---heavy-css-performance-issues

[changeset-bot]

🦋 Changeset detected

Latest commit: cfaea07

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@db-ux/core-components	Patch
@db-ux/ngx-core-components	Patch
@db-ux/react-core-components	Patch
@db-ux/wc-core-components	Patch
@db-ux/v-core-components	Patch
@db-ux/core-foundations	Patch
@db-ux/core-stylelint	Patch
@db-ux/core-migration	Patch
@db-ux/agent-cli	Patch

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Pull request overview

This PR addresses CSS performance issues by simplifying expensive selector patterns in the foundation styles. The changes remove performance-costly selectors while maintaining the same functionality.

• Removed expensive :has() pseudo-class selector for color-scheme detection
• Simplified box-sizing pattern from inheritance-based to direct assignment