When /background was sent during an active run, it was not in the
platform adapter's bypass list and fell through to the interrupt path
instead of spawning a parallel background task.
Add "background" to the active-session command bypass in the platform
adapter, and add an early return in the gateway runner's running-agent
guard to route /background to _handle_background_command() before it
reaches the default interrupt logic.
Fixes#6827
429da6cbce