[app-server] introduce turn/completed v2 event

[celia-oai]

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"
<     }
<   }
< }

[chatgpt-codex-connector]

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[owenlin0]

lgmt overall! can we update the turn_start.rs and turn_interrupt.rs tests to check turn/completed?