avoid error when /compact response has no token_usage (#2417) (#2640)

**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>

codex-rs/core/src/codex.rs

@@ -2816,15 +2816,9 @@ async fn drain_to_completed(
                 response_id: _,
                 token_usage,
             }) => {
-                let token_usage = match token_usage {
-                    Some(usage) => usage,
-                    None => {
-                        return Err(CodexErr::Stream(
-                            "token_usage was None in ResponseEvent::Completed".into(),
-                            None,
-                        ));
-                    }
-                };
+                // some providers don't return token usage, so we default
+                // TODO: consider approximate token usage
+                let token_usage = token_usage.unwrap_or_default();
                 sess.tx_event
                     .send(Event {
                         id: sub_id.to_string(),
@@ -2832,6 +2826,7 @@ async fn drain_to_completed(
                     })
                     .await
                     .ok();
+
                 return Ok(());
             }
             Ok(_) => continue,