Interface ErrorResponderTarget

The minimal context an error responder needs to write a response: the request-scoped state (where the responder itself is published), the response builder to write to, and the request (for the Problem Details instance).

IRequestContext is structurally assignable to this, so middleware passes its full context while the kernel's pre-pipeline sites — the shutdown drain and the malformed-request 400, which run before a context exists, and the request-lifecycle hooks, which run before the pipeline — pass a bare object or a freshly created context. The kernel seeds the application's resolved responder into that state (from the pipeline's brand, see ERROR_RESPONDER_BRAND) before the site runs, so even a pre- pipeline site answers in the configured format when an errorHandler is registered; without one, the state carries no responder and the fallback shape is written.