Fix exec TurnStartParams initializer

Add the new ephemeral_context field to codex-exec's turn/start request so the crate compiles against the updated app-server protocol.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec/src/lib.rs

@@ -685,6 +685,7 @@ async fn run_exec_session(args: ExecRunArgs) -> anyhow::Result<()> {
                     params: TurnStartParams {
                         thread_id: primary_thread_id_for_span.clone(),
                         input: items.into_iter().map(Into::into).collect(),
+                        ephemeral_context: None,
                         cwd: Some(default_cwd),
                         approval_policy: Some(default_approval_policy.into()),
                         sandbox_policy: Some(default_sandbox_policy.clone().into()),