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/resume_picker.rs

@@ -2639,6 +2639,7 @@ mod tests {
         let thread_id = ThreadId::new();
         let thread = Thread {
             id: thread_id.to_string(),
+            forked_from_id: None,
             preview: String::from("remote thread"),
             ephemeral: false,
             model_provider: String::from("openai"),