app-server: expose loaded thread status via read/list and notifications (#11786)

Motivation
- Today, a newly connected client has no direct way to determine the
current runtime status of threads from read/list responses alone.
- This forces clients to infer state from transient events, which can
lead to stale or inconsistent UI when reconnecting or attaching late.

Changes
- Add `status` to `thread/read` responses.
- Add `statuses` to `thread/list` responses.
- Emit `thread/status/changed` notifications with `threadId` and the new
status.
- Track runtime status for all loaded threads and default unknown
threads to `idle`.
- Update protocol/docs/tests/schema fixtures for the revised API.

Testing
- Validated protocol API changes with automated protocol tests and
regenerated schema/type fixtures.
- Validated app-server behavior with unit and integration test suites,
including status transitions and notifications.

codex-rs/app-server/tests/suite/v2/thread_read.rs

@@ -12,10 +12,14 @@ use codex_app_server_protocol::ThreadReadParams;
 use codex_app_server_protocol::ThreadReadResponse;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadStartResponse;
+use codex_app_server_protocol::ThreadStatus;
+use codex_app_server_protocol::TurnStartParams;
+use codex_app_server_protocol::TurnStartResponse;
 use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
 use codex_protocol::user_input::ByteRange;
 use codex_protocol::user_input::TextElement;
+use core_test_support::responses;
 use pretty_assertions::assert_eq;
 use std::path::Path;
 use std::path::PathBuf;
@@ -73,6 +77,7 @@ async fn thread_read_returns_summary_without_turns() -> Result<()> {
     assert_eq!(thread.source, SessionSource::Cli);
     assert_eq!(thread.git_info, None);
     assert_eq!(thread.turns.len(), 0);
+    assert_eq!(thread.status, ThreadStatus::NotLoaded);
 
     Ok(())
 }
@@ -133,6 +138,7 @@ async fn thread_read_can_include_turns() -> Result<()> {
         }
         other => panic!("expected user message item, got {other:?}"),
     }
+    assert_eq!(thread.status, ThreadStatus::NotLoaded);
 
     Ok(())
 }
@@ -181,6 +187,7 @@ async fn thread_read_loaded_thread_returns_precomputed_path_before_materializati
     assert_eq!(read.path, Some(thread_path));
     assert!(read.preview.is_empty());
     assert_eq!(read.turns.len(), 0);
+    assert_eq!(read.status, ThreadStatus::Idle);
 
     Ok(())
 }
@@ -236,6 +243,73 @@ async fn thread_read_include_turns_rejects_unmaterialized_loaded_thread() -> Res
     Ok(())
 }
 
+#[tokio::test]
+async fn thread_read_reports_system_error_idle_flag_after_failed_turn() -> Result<()> {
+    let server = responses::start_mock_server().await;
+    let _response_mock = responses::mount_sse_once(
+        &server,
+        responses::sse_failed("resp-1", "server_error", "simulated failure"),
+    )
+    .await;
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let start_id = mcp
+        .send_thread_start_request(ThreadStartParams {
+            model: Some("mock-model".to_string()),
+            ..Default::default()
+        })
+        .await?;
+    let start_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(start_id)),
+    )
+    .await??;
+    let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(start_resp)?;
+
+    let turn_start_id = mcp
+        .send_turn_start_request(TurnStartParams {
+            thread_id: thread.id.clone(),
+            input: vec![UserInput::Text {
+                text: "fail this turn".to_string(),
+                text_elements: Vec::new(),
+            }],
+            ..Default::default()
+        })
+        .await?;
+    let turn_start_response: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(turn_start_id)),
+    )
+    .await??;
+    let _: TurnStartResponse = to_response::<TurnStartResponse>(turn_start_response)?;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("error"),
+    )
+    .await??;
+
+    let read_id = mcp
+        .send_thread_read_request(ThreadReadParams {
+            thread_id: thread.id,
+            include_turns: false,
+        })
+        .await?;
+    let read_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(read_id)),
+    )
+    .await??;
+    let ThreadReadResponse { thread } = to_response::<ThreadReadResponse>(read_resp)?;
+
+    assert_eq!(thread.status, ThreadStatus::SystemError,);
+
+    Ok(())
+}
+
 // Helper to create a config.toml pointing at the mock model server.
 fn create_config_toml(codex_home: &Path, server_uri: &str) -> std::io::Result<()> {
     let config_toml = codex_home.join("config.toml");