Add configurable "toxics" to proxies

Add configurable "toxics" to proxies: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Right now a proxy is a dumb pipe: whatever a client sends is forwarded verbatim to the upstream and back. We want to be able to deliberately degrade a proxied connection so we can…