WebApp interface

Minimal application surface needed by configureResponseController to register the SDK-specific Activity callback POST route. Express's Application structurally satisfies this. Framework integrations that do not satisfy it should wrap createAgentResponseHandler; Fastify users should use configureResponseController from @microsoft/agents-hosting-fastify.

WebApp is a minimal structural shape rather than a richer, named route-registrar contract. It is exported so it has a stable name in the generated type declarations and API report (it appears in the exported configureResponseController signature). Any framework app whose post(path, handler) method structurally matches satisfies it.