Expose lifecycle timestamps on app-server thread items (#57172)

## What changed

- Add nullable `startedAtMs` and `completedAtMs` fields to `ThreadItem` in live notifications and turn history. Values are Unix milliseconds when recorded.
- Preserve recorded item times through history reconstruction, pagination, and resume, including command start times recorded during approval.
- Separate the Rust item payload into `ThreadItemPayload` while keeping the JSON payload flat, and update consumers, schemas, and generated TypeScript and Python types.

## Testing

Extend coverage for timestamp serialization, history reconstruction, paginated reads and resume, and persisted Python SDK items.

GitOrigin-RevId: ecbcee95af52db7c807b46a6e9deb30e5fba1f80

codex-rs/app-server-protocol/src/protocol/v2/thread_data.rs

@@ -1,6 +1,7 @@
 use super::CodexErrorInfo;
 use super::ThreadEnvironment;
 use super::ThreadItem;
+use super::ThreadItemPayload;
 use super::ThreadStatus;
 use super::TurnStatus;
 use crate::JsonSchema;
@@ -409,6 +410,15 @@ pub struct Turn {
     pub duration_ms: Option<i64>,
 }
 
+impl Turn {
+    /// Iterates over the item payloads in this turn.
+    pub fn item_payloads(
+        &self,
+    ) -> impl DoubleEndedIterator<Item = &ThreadItemPayload> + ExactSizeIterator {
+        self.items.iter().map(|item| &item.payload)
+    }
+}
+
 #[derive(Default, Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]