fix(nuxt): preserve multiple set-cookie headers - #9574

Description

Fixes #9573

authenticateRequest() can return multiple Set-Cookie directives while completing a handshake or refreshing a session. The Nuxt middleware previously forwarded each directive with H3's setResponseHeader(), so every cookie replaced the one before it and only the final directive reached the browser.

This change collects all authentication Set-Cookie values and forwards them to H3 as one array-valued response header. Other authentication headers keep the existing replacement behavior. The regression test demonstrates that both a handshake deletion and refreshed session cookie survive the middleware, while an ordinary auth header is still forwarded.

Run pnpm --filter @clerk/nuxt test to execute the regression and Nuxt type checks. The package suite passes 20 tests, and pnpm build passes all 24 monorepo build tasks on Node 24.15.0. A monorepo-wide pnpm test run completed all Nuxt tests but had two unrelated @clerk/ui tests time out under parallel load; both passed when rerun directly (33 passed, 2 skipped, 2 todo).

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other: