Support cancelling an in-flight client.post() call

Support cancelling an in-flight client.post() call: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). I'm using crosspost from a CLI/service that fans a single message out to several networks at once (Bluesky, Mastodon, Twitter, Discord, LinkedIn). It works great in…