Bun.serve: apply TCP backpressure to a request body the handler reads slowly - #36006

`await Bun.write(file, req)` in a `Bun.serve` handler now hangs for any body > 1 MiB. That path (Blob.rs:5232-5250, `BodyValue::Locked` → `WriteFileWaitFromLockedValueTask`) sets `locked.on_receive_value` and awaits the last chunk without calling `on_start_buffering` or creating a ByteStream, so `REQUEST_BODY_BUFFER_ALL` is never set, the pre-stream HWM check pauses the socket at 1 MiB, and nothing ever resumes it — same bug class as the `buffer_action` deadlock adf72bd fixed, on the `on_receive

The fix in adf72bd added `resume_request_body_socket()` to `end_already_responded_stream()`, but this function's own doc comment (lines 1170-1178) states that `self.resp` may point to a socket uSockets has **already freed** via `us_internal_free_closed_sockets` and must be released *without dereferencing it*. When `REQUEST_BODY_PAUSED` is set, `resp.resume_()` → `us_socket_resume(s)` reads `s->fin_deferred` (socket.c:852, libuv) / `s->flags.is_paused` (:857) on freed memory, and `HttpResponse::r