Consume ai-title from external sessions and add end marker (#20261)

## Summary
- Support Claude Code `ai-title` / `aiTitle` records when detecting and
importing external agent sessions.
- Preserve existing `custom-title` / `customTitle` precedence; only fall
back to `aiTitle` when no custom title is present.
- Add coverage for both detection and import title selection, including
the custom-title-over-ai-title case.

## Testing
- `cargo test -p codex-external-agent-sessions`
- `just fix -p codex-external-agent-sessions`

codex-rs/app-server/tests/suite/v2/external_agent_config.rs

@@ -313,7 +313,17 @@ async fn external_agent_config_import_creates_session_rollouts() -> Result<()> {
     .await??;
     let response: ThreadReadResponse = to_response(response)?;
     assert_eq!(response.thread.turns.len(), 1);
-    assert_eq!(response.thread.turns[0].items.len(), 2);
+    let items = &response.thread.turns[0].items;
+    assert_eq!(items.len(), 3);
+    assert_eq!(
+        items.last(),
+        Some(&ThreadItem::AgentMessage {
+            id: "item-3".into(),
+            text: "<EXTERNAL SESSION IMPORTED>".into(),
+            phase: None,
+            memory_citation: None,
+        })
+    );
 
     let request_id = mcp
         .send_thread_resume_request(ThreadResumeParams {