A failed read consumes and closes the request body. Endpoints that read
it a second time then answered "http: invalid Read on closed Body"
instead of the cause: /v1/video/generations, and audio transcriptions
and translations. The first failure is now kept for the rest of the
request, so every later reader reports it.

A body over MAX_REQUEST_BODY_MB now gets 413 instead of 400 on the
endpoints that still answered 400: ordinary relay endpoints such as
/v1/chat/completions, Midjourney, task plugin routes and /v1/tasks.
Task plugin routes also write the cause to the log, and Midjourney
errors no longer repeat "Invalid request:".