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/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -1406,6 +1406,7 @@
       "type": "string"
     },
     "ThreadItem": {
+      "description": "A thread item and its recorded lifecycle times. The payload stays flat on the wire.",
       "oneOf": [
         {
           "properties": {
@@ -2308,7 +2309,28 @@
           "title": "ContextCompactionThreadItem",
           "type": "object"
         }
-      ]
+      ],
+      "properties": {
+        "completedAtMs": {
+          "default": null,
+          "description": "Unix timestamp in milliseconds when the item completed, if recorded.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "startedAtMs": {
+          "default": null,
+          "description": "Unix timestamp in milliseconds when the item started, if recorded.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        }
+      },
+      "type": "object"
     },
     "ThreadSection": {
       "description": "An independently persisted, user-visible thread section.",