fix(design-system): keep Alert variant text foreground explicit (supabase#46050)

## What kind of change does this PR introduce?

Bug fix. Follow-up to DEPR-551, supabase#45302, supabase#45535, supabase#45618, and stacked on
supabase#46049.

## What is the current behaviour?

Warning and destructive Alert variants do not explicitly own their
foreground text colour. In muted parent contexts, nested Admonition
content can inherit lighter text styling.

## What is the new behaviour?

Warning and destructive Alert variants now include `text-foreground`,
just like the default counterpart.

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## Summary by CodeRabbit

* **Style**
* Updated styling for alert components with destructive and warning
variants.

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Author: dnywh

packages/ui/src/components/shadcn/ui/alert.tsx

@@ -16,9 +16,9 @@ export const alertVariants = cva(
         default:
           'bg-surface-200/25 border-default text-foreground [&>svg]:text-background [&>svg]:bg-foreground',
         destructive:
-          'bg-destructive-200 border-destructive-400 [&>svg]:text-destructive-200 [&>svg]:bg-destructive-600',
+          'bg-destructive-200 border-destructive-400 text-foreground [&>svg]:text-destructive-200 [&>svg]:bg-destructive-600',
         warning:
-          'bg-warning-200 border-warning-400 [&>svg]:text-warning-200 [&>svg]:bg-warning-600',
+          'bg-warning-200 border-warning-400 text-foreground [&>svg]:text-warning-200 [&>svg]:bg-warning-600',
       },
     },
     defaultVariants: {