chore: add small debug client (#8894)

Small debug client, do not use in production

codex-rs/debug-client/src/state.rs

@@ -0,0 +1,28 @@
+use std::collections::HashMap;
+
+use codex_app_server_protocol::RequestId;
+
+#[derive(Debug, Default)]
+pub struct State {
+    pub pending: HashMap<RequestId, PendingRequest>,
+    pub thread_id: Option<String>,
+    pub known_threads: Vec<String>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PendingRequest {
+    Start,
+    Resume,
+    List,
+}
+
+#[derive(Debug, Clone)]
+pub enum ReaderEvent {
+    ThreadReady {
+        thread_id: String,
+    },
+    ThreadList {
+        thread_ids: Vec<String>,
+        next_cursor: Option<String>,
+    },
+}