The PSR-15-style contract for a single middleware step: `process(MessageInterface $message, MessageHandlerInterface $handler): ResultInterface`. Implementations decide whether/when to call $handler->handle() and can act on the message or result before/after doing so.

MiddlewarePipelineInterface extends both MiddlewareInterface and MessageHandlerInterface, and adds pipe(MiddlewareInterface $middleware): void for registering middleware.

MiddlewarePipe is the default implementation, backed by an SplQueue. Calling handle() runs the queued middleware against an EmptyPipelineHandler as the terminal handler (see below). Cloning a MiddlewarePipe performs a deep clone of its internal queue so pipelines can be reused safely.

Internal, single-use handler that walks the middleware queue one step at a time. Each call to handle() dequeues the next middleware, clones itself for the remaining queue, and invokes the middleware with that clone as its $handler. Once the queue is exhausted it delegates to the configured EmptyPipelineHandler. Calling handle() on the same Next instance twice throws Exception\NextHandlerAlreadyCalledException.

The library's built-in "terminal" middleware, registered by ConfigProvider with the lowest default priority so it runs last. It uses a MessageHandlerResolverInterface to resolve and invoke the handler mapped to the incoming message, then forwards the resulting ResultInterface on to the next handler in the pipeline so any middleware that runs after it (e.g. logging) can inspect the result.

The default terminal handler used when a MiddlewarePipe's queue is exhausted. If the message it receives is already a ResultInterface (i.e. a result produced by MessageHandlerMiddleware), it returns it as-is; otherwise it throws Exception\MessageException, since no handler ever produced a result.