sandbox-server Gateway and V1 routing for self-hosted deployments + agent-canvas backend support

Will sandbox-server support V1 routing from a single endpoint to the agent server (implement a gateway), or will it only support local mode (with agent-canvas limited to working on custom ports at `http://localhost`)?

Issue: https://github.com/OpenHands/OpenHands/issues/13504
Ref: https://github.com/OpenHands/OpenHands/pull/14909
