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/src/protocol/v2.rs

@@ -2434,6 +2434,8 @@ pub struct ThreadForkParams {
     pub base_instructions: Option<String>,
     #[ts(optional = nullable)]
     pub developer_instructions: Option<String>,
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub ephemeral: bool,
     /// If true, persist additional rollout EventMsg variants required to
     /// reconstruct a richer thread history on subsequent resume/fork/read.
     #[experimental("thread/fork.persistFullHistory")]