fix: preserve session cookie priority

[lprnmns]

The public CookieOptions type accepts the cookie priority option, but the internal session Cookie model dropped that value before calling reply.setCookie. As a result, valid low, medium, and high values never produced a Priority attribute.

This preserves priority alongside the existing cookie properties and includes it in the serialized cookie options. A focused integration test verifies that priority: 'high' emits Priority=High.

Validation:

• node --test --test-name-pattern='should set session cookie with priority' test/cookie.test.js
• npm run test && npm run benchmark --if-present
• npm run lint

The full suite passes with 129 unit tests, 47 TypeScript assertions, and 100% reported coverage. The change is backward-compatible for existing configurations: an undefined priority remains omitted, while already-typed valid values now reach the underlying cookie serializer.

No issue is linked; this was independently reproduced on current main.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added (the existing public type already exposes this option)
• commit message and code follows the Developer's Certificate of Origin and the Code of conduct