Image generation requests (forwardOpenAIImagesOAuth and
forwardOpenAIImagesAPIKey) were calling detachStreamUpstreamContext with
parsed.Stream, which for non-streaming requests (Stream=false) simply
returned the original client context unchanged. When the client
disconnected before the upstream completed (30-80s for image gen), the
context cancellation propagated to the upstream HTTP request, causing a
502 error despite the upstream having already started processing.
Switch to detachUpstreamContext (unconditional detach) so the upstream
image generation request is always bound to a background context and
completes regardless of client lifecycle.
Fixes#2310
