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/v2.rs

@@ -3574,6 +3574,8 @@ impl From<CoreSkillErrorInfo> for SkillErrorInfo {
 #[ts(export_to = "v2/")]
 pub struct Thread {
     pub id: String,
+    /// Source thread id when this thread was created by forking another thread.
+    pub forked_from_id: Option<String>,
     /// Usually the first user message in the thread, if available.
     pub preview: String,
     /// Whether the thread is ephemeral and should not be materialized on disk.