Preserve and expose thread item lifecycle timestamps (#47114)

## What changed

- Persist producer-recorded item start and completion times in thread history, preserving existing non-null values on updates.
- Return nullable `startedAtMs` and `completedAtMs` Unix millisecond timestamps from `thread/items/list`, and update the generated schemas and TypeScript and Python types.
- Leave timestamps unknown for existing rows and older writers.

## Testing

Add coverage for exact timestamp persistence, protocol round trips with present and missing timestamps, paginated item responses, and compatibility with older database writers.

GitOrigin-RevId: 894853481085782ff54db1704799e41446d15e51

sdk/python/src/openai_codex/generated/v2_all.py

@@ -11194,7 +11194,21 @@ class ThreadItemEntry(BaseModel):
     model_config = ConfigDict(
         populate_by_name=True,
     )
+    completed_at_ms: Annotated[
+        int | None,
+        Field(
+            alias="completedAtMs",
+            description="Unix timestamp (milliseconds) when the item completed, if recorded by the producer.",
+        ),
+    ] = None
     item: ThreadItem
+    started_at_ms: Annotated[
+        int | None,
+        Field(
+            alias="startedAtMs",
+            description="Unix timestamp (milliseconds) when the item started, if recorded by the producer.",
+        ),
+    ] = None
     turn_id: Annotated[str, Field(alias="turnId", description="Turn containing this item.")]