RustUpstreamError has one constructor that always sets headers, argument
checks raise typed errors mapped once, OCR asks upstream_response(), the
Responses stream error event comes from the error type, gateway-inference
loses its string variants, and auth faults classify through one
litellm_auth predicate on every route.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Messages describe their own layer and every cause is a typed source,
predicates match exhaustively, and boundaries render with
litellm_error::Report. Media downloads no longer count as dispatched,
Error::upstream bounds the provider body itself, and a forwarded
header SigV4 cannot sign is the caller's fault.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>