feat: ephemeral threads (#9765)

Add ephemeral threads capabilities. Only exposed through the
`app-server` v2

The idea is to disable the rollout recorder for those threads.

codex-rs/protocol/src/protocol.rs

@@ -2143,7 +2143,9 @@ pub struct SessionConfiguredEvent {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub initial_messages: Option<Vec<EventMsg>>,
 
-    pub rollout_path: PathBuf,
+    /// Path in which the rollout is stored. Can be `None` for ephemeral threads
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub rollout_path: Option<PathBuf>,
 }
 
 /// User's decision in response to an ExecApprovalRequest.
@@ -2495,7 +2497,7 @@ mod tests {
                 history_log_id: 0,
                 history_entry_count: 0,
                 initial_messages: None,
-                rollout_path: rollout_file.path().to_path_buf(),
+                rollout_path: Some(rollout_file.path().to_path_buf()),
             }),
         };