Fix fork source display in /status (expose forked_from_id in app server) (#16596)

Addresses #16560

Problem: `/status` stopped showing the source thread id in forked TUI
sessions after the app-server migration.

Solution: Carry fork source ids through app-server v2 thread data and
the TUI session adapter, and update TUI fixtures so `/status` matches
the old TUI behavior.

codex-rs/tui/src/app/app_server_adapter.rs

@@ -1251,6 +1251,7 @@ mod tests {
     fn replays_command_execution_items_from_thread_snapshots() {
         let thread = Thread {
             id: "019cee8c-b993-7e33-88c0-014d4e62612d".to_string(),
+            forked_from_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),
@@ -1417,6 +1418,7 @@ mod tests {
         let events = thread_snapshot_events(
             &Thread {
                 id: thread_id.to_string(),
+                forked_from_id: None,
                 preview: "hello".to_string(),
                 ephemeral: false,
                 model_provider: "openai".to_string(),

codex-rs/tui/src/app/loaded_threads.rs

@@ -111,6 +111,7 @@ mod tests {
     fn test_thread(thread_id: ThreadId, source: SessionSource) -> Thread {
         Thread {
             id: thread_id.to_string(),
+            forked_from_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),