Report completed sub-agent activity on parent turns (#40437)

## Why

A Multi-Agent V2 child can finish after the parent turn that spawned it, so its
successful completion needs to remain associated with that parent turn in
notifications and history.

## What changed

- Add `completed` to `SubAgentActivityKind` and emit the corresponding started
  and completed item events on the spawning parent turn when a child succeeds.
- Persist completed activity as a canonical turn item in both legacy and
  paginated history, and expose it through app-server history and schemas.
- Render completed activity as terminal in the TUI without counting it as a
  separate tool call or tool-runtime event.

## Testing

- Cover successful completion in legacy and paginated history, late updates to
  completed parent turns, TUI rendering, analytics, and rollout tracing.
- Verify that terminal child errors do not emit completed activity.

GitOrigin-RevId: 6c71eb8b10e7327611c6af84f70e294e27d94d02

codex-rs/rollout-trace/src/protocol_event_tests.rs

@@ -50,6 +50,20 @@ fn sub_agent_activity_is_a_terminal_tool_runtime_event() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[test]
+fn completed_sub_agent_activity_is_not_a_tool_runtime_event() -> anyhow::Result<()> {
+    let event = EventMsg::SubAgentActivity(SubAgentActivityEvent {
+        event_id: "child-turn-completed".to_string(),
+        occurred_at_ms: 1234,
+        agent_thread_id: ThreadId::new(),
+        agent_path: AgentPath::try_from("/root/reviewer").map_err(anyhow::Error::msg)?,
+        kind: SubAgentActivityKind::Completed,
+    });
+
+    assert!(tool_runtime_trace_event(&event).is_none());
+    Ok(())
+}
+
 #[test]
 fn exec_command_trace_payloads_use_inferred_native_cwd() -> anyhow::Result<()> {
     // Convention inference depends on the URI spelling, not the test host, so exercise both