feat: honor foreign public-route metadata

[kauandotnet]

Summary

• add opt-in interop.publicKeys module configuration
• let BetterAuthGuard honor public-route metadata owned by another guard
• skip session resolution for matching public routes
• preserve fail-closed precedence:
  • handler-level Better Auth requirements override inherited foreign markers
  • explicit handler-level foreign markers win across every configured key
• preserve the guard's existing two-argument constructor
• document and export the new interop option

Why

Applications integrating another framework with its own @Public() metadata currently need an application-level wrapper guard. This makes that interoperability first-class while preserving Better Auth's authorization precedence.

Public API

BetterAuthModuleOptions gains the optional interop.publicKeys setting. Existing configuration and direct two-argument BetterAuthGuard construction remain compatible.

Validation

• pnpm test — 155 Express and 155 Fastify tests passed
• pnpm run check
• pnpm run verify:pack
• packed-type consumer, including the original guard constructor
• git diff --check