**Context**
When running `/compact`, `drain_to_completed` would throw an error if
`token_usage` was `None` in `ResponseEvent::Completed`. This made the
command fail even though everything else had succeeded.
**What changed**
- Instead of erroring, we now just check `if let Some(token_usage)`
before sending the event.
- If it’s missing, we skip it and move on.
**Why**
This makes `AgentTask::compact()` behave in the same way as
`AgentTask::spawn()`, which also doesn’t error out when `token_usage`
isn’t available. Keeps things consistent and avoids unnecessary
failures.
**Fixes**
Closes#2417
---------
Co-authored-by: Ahmed Ibrahim <aibrahim@openai.com>
