Add eager auth for synchronous token access

[nicknisi]

Summary

• Add eagerAuth middleware option for synchronous access token availability
• Implement 30-second JWT cookie mechanism for third-party service integration
• Add comprehensive test coverage (100% for tokenStore and cookie modules)
• Include security documentation and best practices
• Fix cookie name consistency across codebase

Key Changes

• New eagerAuth option: Enables synchronous token access on initial page load
• JWT cookie mechanism: 30-second TTL cookie automatically consumed by client
• Request type detection: Only sets cookies on document requests, not API/RSC/prefetch

Usage

To opt-in to this feature, add the following in middleware.ts:

import { authkitMiddleware } from "@workos-inc/authkit-nextjs";

export default authkitMiddleware({
  eagerAuth: true
});

With this enabled, the token will now be immediately accessible via the useAccessToken hook on the client.

Without that, the first couple of renders will not have the accessToken available until it is asynchronously loaded.

Fixes #286

[cmatheson]

i'm slightly uncomfortable that we're having to string interpolate a regexp here... can we do some light-weight parsing or something instead?

[dandorman]

Nice work! (and very thorough job on the tests)