Chatbot template: UI for background mode

[bbqiu]

stacked on top of #146

• follows the client contract in the readme of the last PR to enable UI backend request to agent backend with background=True

manual testing WIP

[bbqiu]

i realized that to support long running stuff within a UI that is colocated with the agent (on the same app), we do not need to do anything special bc:

user/UI frontend -1-> UI backend -2-> Agent

when UI + agent are colocated, connection 2 is never broken, so you can continue to run things without a problem. this PR will only be needed once we support querying the agentic loop w/ background mode=True, so closing this PR for now

changes required for future change:

1. send a request with background=True, stream=True by default, if it fails, fall back to stream=True, and then stream=False
2. after connection 2 times out, the protocol for resumption should be the GET /retrieve endpoint.
   a. there should probably be a tracker of last emitted event timestamp and to stop polling if the stream has gone stale

another working reference: https://github.com/david-tempelmann/agent-longrunning-openai-agents-sdk/pull/1