[hoanle0126]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Description

Fixes #158

Marks err as optional in CallbackSession (type CallbackSession = (err?: any, result?: Fastify.Session | null) => void, matching Callback) so custom SessionStore.get implementations can invoke callback() without arguments when a session is not found, as well as callback(null), callback(null, null), callback(error), and callback(null, sessionData).

Adds type tests in types/index.tst.ts covering custom SessionStore callback invocations.