Problem

Route shorthand hook options were instantiated from *MetaHookHandler with the default Return type (void).

That makes eslint @typescript-eslint/no-misused-promises report async hook references like:

const preHandler = async (request: FastifyRequest) => {}
fastify.get("/", { preHandler }, async () => "ok")

Changes

• Pass void | Promise<unknown> as the Return generic for route-level hooks in RouteShorthandOptions:
  • onRequest, preParsing, preValidation, preHandler, preSerialization, onSend, onResponse, onTimeout, onError, onRequestAbort
• Add a type test in test/types/route.test-d.ts for async preHandler passed by reference

Notes

This is separate from #5498 / SafePromiseLike (no-floating-promises) and addresses no-misused-promises contextual typing in route hook options.

Closes #6513