Introduce fluent Response API and strict dispatcher return contract #470

Summary

Make the HTTP response API consistently fluent and require route actions / closures to return Response.

Goals

• allow return $response->json(...)
• allow return $response->html(...)
• require route actions and closures to return Response
• fail fast on invalid return types

In scope

• src/Http/Traits/Response/Body.php
• src/Http/Traits/Response/Header.php
• src/Http/Traits/Response/Status.php
• src/Router/RouteDispatcher.php
• route dispatcher tests

Required API changes

These methods should return Response:

• json()
• html()
• xml()
• jsonp()
• setHeader()
• setStatusCode()
• set()

Dispatcher contract

• action return type must be Response
• closure route return type must be Response
• invalid non-Response return values should raise an exception

Acceptance criteria

• fluent response builder API is available
• route actions can return response builder calls directly
• dispatcher returns the produced Response
• invalid action / closure return values fail fast

Validation

• route dispatcher unit tests
• response trait unit tests
• composer test
• composer phpstan