Fix for empty response body when debugging unseekable streams

[JonPurvis]

Fixes #447

When debug mode is on, the default response debugger calls $response->body() to dump the response. If the PSR-7 body stream is unseekable (e.g. many LLM/OpenAI APIs), that first read consumes the stream and it is never rewound, so later calls to body() or json() return empty.

This change:

1. Response::withBufferedBody(string $body): static
   Returns a new Response with the same metadata but the body replaced by a seekable stream containing the given string (using ResponseInterface::withBody() and GuzzleHttp\Psr7\Utils::streamFor()).

2. HasDebugging::debugResponse()
   In the response middleware: if the PSR body stream is not seekable, we read the body once, build a replacement response via withBufferedBody(), run the debug callback on that response (so the dump still shows the full body), and return the replacement response from the middleware so the rest of the pipeline and the caller receive a response whose body can still be read. If the stream is seekable, behavior is unchanged (same response, no buffering).