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/app-server-protocol/src/protocol/common.rs

@@ -1307,6 +1307,7 @@ mod tests {
             response: v2::ThreadStartResponse {
                 thread: v2::Thread {
                     id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
+                    forked_from_id: None,
                     preview: "first prompt".to_string(),
                     ephemeral: true,
                     model_provider: "openai".to_string(),
@@ -1343,6 +1344,7 @@ mod tests {
                 "response": {
                     "thread": {
                         "id": "67e55044-10b1-426f-9247-bb680e5fe0c8",
+                        "forkedFromId": null,
                         "preview": "first prompt",
                         "ephemeral": true,
                         "modelProvider": "openai",