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/codex_app_server_protocol.schemas.json

@@ -15137,6 +15137,9 @@
               "null"
             ]
           },
+          "ephemeral": {
+            "type": "boolean"
+          },
           "model": {
             "description": "Configuration overrides for the forked thread, if any.",
             "type": [