Provide declarative AOT binding type metadata for WebMvc.fn routes

[mingchiuli]

Proposal

Provide declarative AOT binding type metadata for WebMvc.fn routes.

WebMvc.fn handler functions expose only ServerRequest and ServerResponse.
As a result, request and response body types are not visible in handler method
signatures and cannot be discovered in the same way as annotated controllers.

Applications currently need to maintain a separate list such as:

@RegisterReflectionForBinding({
    CreateRequest.class,
    Result.class,
    UserResponse.class
})

This is easy to forget when adding or changing a route.

Would Spring Framework consider adding a typed declaration mechanism to the
WebMvc.fn router or response builder, allowing request and response binding
types (including ParameterizedTypeReference) to be associated with a route
and processed automatically during AOT?

The goal is not to analyze arbitrary handler bytecode, but to provide an
official declarative API colocated with each functional route.

This could potentially also be considered for WebFlux.fn for API parity.

[adityaanikam]

This is a real, well-scoped gap — WebMvc.fn handlers only ever expose ServerRequest/ServerResponse to the router, so there's no route-local place to declare the actual body types, and @RegisterReflectionForBinding lists end up hand-maintained and drifting from the routes as you described.

Since this is a new API surface rather than a bug fix, I'd rather get a maintainer's read on the shape before writing anything — specifically whether this should live as metadata on the RouterFunction/RequestPredicate builder chain itself, or as a separate declarative registry keyed by route, since that decision affects whether ParameterizedTypeReference support composes cleanly with the existing builder API. Happy to prototype either direction once there's a steer on which fits Spring's existing AOT contribution model better.