app-server: improve thread resume rejoin flow (#11776)

thread/resume response includes latest turn with all items, in band so
no events are stale or lost

Testing
- e2e tested using app-server-test-client using flow described in
"Testing Thread Rejoin Behavior" in
codex-rs/app-server-test-client/README.md
- e2e tested in codex desktop by reconnecting to a running turn

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -2727,6 +2727,26 @@ pub enum ThreadItem {
     ContextCompaction { id: String },
 }
 
+impl ThreadItem {
+    pub fn id(&self) -> &str {
+        match self {
+            ThreadItem::UserMessage { id, .. }
+            | ThreadItem::AgentMessage { id, .. }
+            | ThreadItem::Plan { id, .. }
+            | ThreadItem::Reasoning { id, .. }
+            | ThreadItem::CommandExecution { id, .. }
+            | ThreadItem::FileChange { id, .. }
+            | ThreadItem::McpToolCall { id, .. }
+            | ThreadItem::CollabAgentToolCall { id, .. }
+            | ThreadItem::WebSearch { id, .. }
+            | ThreadItem::ImageView { id, .. }
+            | ThreadItem::EnteredReviewMode { id, .. }
+            | ThreadItem::ExitedReviewMode { id, .. }
+            | ThreadItem::ContextCompaction { id, .. } => id,
+        }
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(tag = "type", rename_all = "camelCase")]
 #[ts(tag = "type", rename_all = "camelCase")]