fix(v4): forward refs in Button asChild variants

[raashish1601]

Summary

Fixes #8930.

• Convert both v4 registry Button variants to React.forwardRef components.
• Forward the ref through Slot.Root when asChild is used, preserving composition with Tooltip and other slot-based primitives.
• Add SSR-focused tests that verify Button asChild renders only the child element and does not wrap it in a nested <button>.

Validation

• corepack pnpm vitest run apps/v4/registry/new-york-v4/ui/button.test.tsx apps/v4/registry/bases/radix/ui/button.test.tsx
• corepack pnpm --filter=v4 typecheck
• corepack pnpm --filter=v4 lint
• corepack pnpm --filter=v4 validate:registries
• corepack pnpm --filter=shadcn build
• git diff --check origin/main...HEAD

Notes

• corepack pnpm --filter=v4 build could not complete in my local Windows environment because the script invokes bare pnpm and this machine only exposes pnpm through Corepack. Attempting the registry step manually with tsx also fails before this change is exercised because the script expects Bun's JSX runtime and Bun is not installed locally.

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