Late aborted Copilot turn can wedge subsequent messages

After aborting while a tool was still running, a late `session.idle` cleared the replacement turn. Later messages were accepted but never produced model, tool, or idle events; reloading the window recovered the session.

```text
11:00:49 Aborting session...
11:00:49 sendMessage called...
11:02:25 tool.execution_complete emitted after cancellation; dropping
11:02:29 Session idle
11:02:29 Reasoning delta emitted with no active turn; dropping
11:02:29 Markdown delta emitted with no active turn; dropping
11:23:09 No event ID found for truncation, nothing to truncate
11:24:16 session.send() returned
```