When the gateway spawned a background agent (e.g. for delegation), media
URLs and types from the originating message weren't forwarded — the bg
agent saw the prompt but no attached images. Vision-enabled tasks
effectively lost their inputs.
Forwards media_urls/media_types through the bg-task spawn path and
runs the same vision-enrichment step the main flow uses, so the bg
agent gets image descriptions inlined into its prompt.
Closes#25614.
Salvage of #25603 by @oxngon (manually re-applied — original branch
was severely stale against current main).
3adde245b7