The ctx.state key under which an application's resolved error responder is published.

IRequestContext.state is a Map<string, unknown> — string keys only — so the seam is a string constant rather than a Symbol.for brand. Exported so a third-party error handler can publish a responder too.

Follows the <owner-package>:<kebab-key> convention documented in state-keys.ts; the owner is @setu-ts/exceptions, whose errorHandler writes it.