similar to logic in
`codex/codex-rs/exec/src/event_processor_with_jsonl_output.rs`.
translation of v1 -> v2 events:
`codex/event/task_complete` -> `turn/completed`
`codex/event/turn_aborted` -> `turn/completed` with `interrupted` status
`codex/event/error` -> `turn/completed` with `error` status
this PR also makes `items` field in `Turn` optional. For now, we only
populate it when we resume a thread, and leave it as None for all other
places until we properly rewrite core to keep track of items.
tested using the codex app server client. example new event:
```
< {
< "method": "turn/completed",
< "params": {
< "turn": {
< "id": "0",
< "items": [],
< "status": "interrupted"
< }
< }
< }
```
For app-server development it's been helpful to be able to trigger some
test flows end-to-end and print the JSON-RPC messages sent between
client and server.
