fix(a11y): improve destructive alert description contrast in v4

[galoi]

Summary

• replace text-destructive/90 with text-destructive for destructive Alert descriptions in new-york-v4
• align the same Alert variant across the generated v4 style files and published registry JSON
• update the style token source files so future registry builds keep the accessible color choice

Why

Issue #10431 reports that the destructive Alert description color sits on the WCAG AA threshold against the default white card background. The current text-destructive/90 value reduces contrast enough that downstream accessibility tooling can report a failure for normal body text.

Root Cause

The destructive Alert variant explicitly applies a 90% alpha destructive text color to the description slot. With the documented default theme tokens, that lowers the effective foreground/background contrast to the edge of the AA minimum.

Impact

Destructive Alert descriptions now use the full destructive token instead of the alpha-reduced variant, giving the component more contrast headroom with the default theme while preserving the existing visual intent.

Validation

• pnpm --filter=shadcn build
• pnpm --filter=v4 lint
• pnpm exec prettier --check 'apps/v4/registry/new-york-v4/ui/alert.tsx' 'apps/v4/registry/styles/style-*.css' 'apps/v4/styles/*/ui/alert.tsx' 'apps/v4/styles/*/ui-rtl/alert.tsx' 'apps/v4/public/r/styles/*/alert.json'

Closes #10431

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