Fixes #4685

This PR restores the ability for developers to provide an explicit return type via a generic type argument on RouteHandlerMethod and related routing shorthand methods. In Fastify v4, the generic return type was removed in favor of strict type inference using ResolveFastifyReplyReturnType, which unfortunately broke community plugins (like fastify-funky) and custom hooks that needed to extend or override the return type signature of route handlers.

To fix this while maintaining v4's strict default type inference, this PR introduces a new ResolveReply generic type parameter. It is placed at the end of the generics list and defaults directly to ResolveFastifyReplyReturnType<TypeProvider, SchemaCompiler, RouteGeneric>. This ensures total backward compatibility for users who don't supply the parameter, while providing the requested escape hatch for those who do.

Checklist

• run npm run test && npm run benchmark --if-present
• 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