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/schema/json/v2/ThreadResumeResponse.json

@@ -1042,6 +1042,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {