Follow-up on top of the helix4u #6392 cherry-pick:
- reuse one helper for actionable Docker-local file-not-found errors
across document/image/video/audio local-media send paths
- include /outputs/... alongside /output/... in the container-local
path hint
- soften the gateway startup warning so it does not imply custom
host-visible mounts are broken; the warning now targets the specific
risky pattern of emitting container-local MEDIA paths without an
explicit export mount
- add focused regressions for /outputs/... and non-document media hint
coverage
This keeps the salvage aligned with the actual MEDIA delivery problem on
current main while reducing false-positive operator messaging.