Do not attempt to append after response.completed (#11402)

Completed responses are fully done, and new response must be created.

codex-rs/codex-api/src/common.rs

@@ -63,6 +63,8 @@ pub enum ResponseEvent {
     Completed {
         response_id: String,
         token_usage: Option<TokenUsage>,
+        /// Whether the client can append more items to a long-running websocket response.
+        can_append: bool,
     },
     OutputTextDelta(String),
     ReasoningSummaryDelta {