Use app server metadata for fork parent titles (#18632)

## Problem
The TUI resolved fork parent titles from local CODEX_HOME metadata,
which could show missing or stale titles when app-server metadata is
authoritative.

This is a lingering bug left over from the migration of the TUI to the
app-server interface. I found it when I asked Codex to review all places
where the TUI code was still directly accessing the local CODEX_HOME.

## Solution
Route fork parent title metadata through the app-server session state
and render only that supplied title, with focused snapshot coverage for
stale local metadata.

## Testing
I manually tested by renaming a thread then forking it and confirming
that the "forked from" message indicated the parent thread's name.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__app_server_forked_thread_history_line.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/chatwidget/tests/history_replay.rs
+expression: combined
+---
+• Thread forked from app-server-parent-thread (e9f18a88-8081-4e51-9d4e-8af5cde2d8dd)

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__app_server_forked_thread_history_line_without_app_server_name.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/chatwidget/tests/history_replay.rs
+expression: combined
+---
+• Thread forked from 019c2d47-4935-7423-a190-05691f566092

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__forked_thread_history_line.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/chatwidget/tests/history_replay.rs
+expression: combined
+---
+• Thread forked from named-thread (e9f18a88-8081-4e51-9d4e-8af5cde2d8dd)

codex-rs/tui/src/chatwidget/tests/history_replay.rs

@@ -387,6 +387,36 @@ async fn replayed_user_message_with_only_local_images_does_not_render_history_ce
     assert!(!found_user_history_cell);
 }
 
+#[tokio::test]
+async fn forked_thread_history_line_includes_name_and_id_snapshot() {
+    let (chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    let mut chat = chat;
+
+    let forked_from_id =
+        ThreadId::from_string("e9f18a88-8081-4e51-9d4e-8af5cde2d8dd").expect("forked id");
+
+    chat.emit_forked_thread_event(forked_from_id, Some("named-thread".to_string()));
+
+    let history_cell = tokio::time::timeout(std::time::Duration::from_secs(2), async {
+        loop {
+            match rx.recv().await {
+                Some(AppEvent::InsertHistoryCell(cell)) => break cell,
+                Some(_) => continue,
+                None => panic!("app event channel closed before forked thread history was emitted"),
+            }
+        }
+    })
+    .await
+    .expect("timed out waiting for forked thread history");
+    let combined = lines_to_single_string(&history_cell.display_lines(/*width*/ 80));
+
+    assert!(
+        combined.contains("Thread forked from"),
+        "expected forked thread message in history"
+    );
+    assert_chatwidget_snapshot!("forked_thread_history_line", combined);
+}
+
 #[tokio::test]
 async fn forked_thread_history_line_without_name_shows_id_once_snapshot() {
     let (chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
@@ -398,7 +428,7 @@ async fn forked_thread_history_line_without_name_shows_id_once_snapshot() {
 
     let forked_from_id =
         ThreadId::from_string("019c2d47-4935-7423-a190-05691f566092").expect("forked id");
-    chat.emit_forked_thread_event(forked_from_id);
+    chat.emit_forked_thread_event(forked_from_id, /*fork_parent_title*/ None);
 
     let history_cell = tokio::time::timeout(std::time::Duration::from_secs(2), async {
         loop {
@@ -416,6 +446,88 @@ async fn forked_thread_history_line_without_name_shows_id_once_snapshot() {
     assert_chatwidget_snapshot!("forked_thread_history_line_without_name", combined);
 }
 
+#[tokio::test]
+async fn app_server_forked_thread_history_line_uses_app_server_title_snapshot() {
+    let (chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    let mut chat = chat;
+    let temp = tempdir().expect("tempdir");
+    chat.config.codex_home =
+        codex_utils_absolute_path::AbsolutePathBuf::from_absolute_path(temp.path())
+            .expect("temp dir is absolute");
+
+    let forked_from_id =
+        ThreadId::from_string("e9f18a88-8081-4e51-9d4e-8af5cde2d8dd").expect("forked id");
+    let session_index_entry = format!(
+        "{{\"id\":\"{forked_from_id}\",\"thread_name\":\"stale-local-thread\",\"updated_at\":\"2024-01-02T00:00:00Z\"}}\n"
+    );
+    std::fs::write(temp.path().join("session_index.jsonl"), session_index_entry)
+        .expect("write session index");
+
+    chat.emit_forked_thread_event(forked_from_id, Some("app-server-parent-thread".to_string()));
+
+    let history_cell = tokio::time::timeout(std::time::Duration::from_secs(2), async {
+        loop {
+            match rx.recv().await {
+                Some(AppEvent::InsertHistoryCell(cell)) => break cell,
+                Some(_) => continue,
+                None => panic!("app event channel closed before forked thread history was emitted"),
+            }
+        }
+    })
+    .await
+    .expect("timed out waiting for forked thread history");
+    let combined = lines_to_single_string(&history_cell.display_lines(/*width*/ 80));
+
+    assert!(combined.contains("app-server-parent-thread"));
+    assert!(
+        !combined.contains("stale-local-thread"),
+        "app-server fork title lookup should not read local CODEX_HOME"
+    );
+    assert_chatwidget_snapshot!("app_server_forked_thread_history_line", combined);
+}
+
+#[tokio::test]
+async fn app_server_forked_thread_history_line_without_app_server_name_ignores_local_snapshot() {
+    let (chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    let mut chat = chat;
+    let temp = tempdir().expect("tempdir");
+    chat.config.codex_home =
+        codex_utils_absolute_path::AbsolutePathBuf::from_absolute_path(temp.path())
+            .expect("temp dir is absolute");
+
+    let forked_from_id =
+        ThreadId::from_string("019c2d47-4935-7423-a190-05691f566092").expect("forked id");
+    let session_index_entry = format!(
+        "{{\"id\":\"{forked_from_id}\",\"thread_name\":\"stale-local-thread\",\"updated_at\":\"2024-01-02T00:00:00Z\"}}\n"
+    );
+    std::fs::write(temp.path().join("session_index.jsonl"), session_index_entry)
+        .expect("write session index");
+
+    chat.emit_forked_thread_event(forked_from_id, /*fork_parent_title*/ None);
+
+    let history_cell = tokio::time::timeout(std::time::Duration::from_secs(2), async {
+        loop {
+            match rx.recv().await {
+                Some(AppEvent::InsertHistoryCell(cell)) => break cell,
+                Some(_) => continue,
+                None => panic!("app event channel closed before forked thread history was emitted"),
+            }
+        }
+    })
+    .await
+    .expect("timed out waiting for forked thread history");
+    let combined = lines_to_single_string(&history_cell.display_lines(/*width*/ 80));
+
+    assert!(
+        !combined.contains("stale-local-thread"),
+        "app-server fork title lookup should not read local CODEX_HOME"
+    );
+    assert_chatwidget_snapshot!(
+        "app_server_forked_thread_history_line_without_app_server_name",
+        combined
+    );
+}
+
 #[tokio::test]
 async fn thread_snapshot_replay_preserves_agent_message_during_review_mode() {
     let (mut chat, mut rx, _ops) = make_chatwidget_manual(/*model_override*/ None).await;

codex-rs/tui/src/chatwidget/tests/side.rs

@@ -9,7 +9,7 @@ async fn forked_thread_history_line_without_name_shows_id_once_snapshot() {
 
     let forked_from_id =
         ThreadId::from_string("019c2d47-4935-7423-a190-05691f566092").expect("forked id");
-    chat.emit_forked_thread_event(forked_from_id);
+    chat.emit_forked_thread_event(forked_from_id, /*fork_parent_title*/ None);
 
     let history_cell = tokio::time::timeout(std::time::Duration::from_secs(2), async {
         loop {