Add ephemeral flag support to thread fork (#14248)

### Summary
This PR adds first-class ephemeral support to thread/fork, bringing it
in line with thread/start. The goal is to support one-off completions on
full forked threads without persisting them as normal user-visible
threads.

### Testing

codex-rs/app-server-protocol/schema/json/v2/ThreadForkParams.json

@@ -100,6 +100,9 @@
         "null"
       ]
     },
+    "ephemeral": {
+      "type": "boolean"
+    },
     "model": {
       "description": "Configuration overrides for the forked thread, if any.",
       "type": [