Expose app-server diagnostics through the experimental API (#37470)

## What changed

- Add the experimental `server/diagnostics` request, returning the app-server process ID, resident memory, platform-dependent physical footprint, and registered diagnostic gauges.
- Export the new protocol types in the generated JSON and TypeScript schemas and document the request.

## Testing

- Verify the response includes process measurements and the registered live-thread gauge.
- Verify the request is rejected unless `capabilities.experimentalApi` is enabled.

GitOrigin-RevId: 099d60a9c421a74ff1a72fe524e0dfeb89cf1057

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

@@ -74,6 +74,7 @@ mod safety_check_downgrade;
 #[cfg(not(target_os = "windows"))]
 mod selected_capability_stack;
 mod selected_environment;
+mod server_diagnostics;
 #[cfg(not(target_os = "windows"))]
 mod session_end;
 mod skills_list;

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

@@ -0,0 +1,100 @@
+use std::time::Duration;
+
+use anyhow::Result;
+use app_test_support::DEFAULT_CLIENT_NAME;
+use app_test_support::MockResponsesConfig;
+use app_test_support::TestAppServer;
+use app_test_support::create_mock_responses_server_repeating_assistant;
+use codex_app_server_protocol::ClientInfo;
+use codex_app_server_protocol::ClientRequest;
+use codex_app_server_protocol::InitializeCapabilities;
+use codex_app_server_protocol::JSONRPCMessage;
+use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::ServerDiagnosticsGauge;
+use codex_app_server_protocol::ServerDiagnosticsParams;
+use codex_app_server_protocol::ServerDiagnosticsResponse;
+use codex_app_server_protocol::ThreadStartParams;
+use pretty_assertions::assert_eq;
+use serde_json::json;
+use tempfile::TempDir;
+use tokio::time::timeout;
+
+const READ_TIMEOUT: Duration = Duration::from_secs(20);
+
+#[tokio::test]
+async fn server_diagnostics_exposes_process_and_registered_thread_gauge() -> Result<()> {
+    let server = create_mock_responses_server_repeating_assistant("Done").await;
+    let codex_home = TempDir::new()?;
+    MockResponsesConfig::new(&server.uri()).write(codex_home.path())?;
+    let mut app_server = TestAppServer::builder()
+        .with_codex_home(codex_home.path())
+        .build_initialized()
+        .await?;
+    app_server
+        .start_thread(ThreadStartParams::default())
+        .await?;
+
+    let diagnostics: ServerDiagnosticsResponse = app_server
+        .request(|request_id| ClientRequest::ServerDiagnostics {
+            request_id,
+            params: ServerDiagnosticsParams::default(),
+        })
+        .await?;
+
+    assert!(diagnostics.process.id > 0);
+    assert!(diagnostics.process.resident_memory_bytes.is_some());
+    #[cfg(target_os = "macos")]
+    assert!(diagnostics.process.physical_footprint_bytes.is_some());
+    #[cfg(not(target_os = "macos"))]
+    assert_eq!(diagnostics.process.physical_footprint_bytes, None);
+    assert_eq!(
+        diagnostics.gauges,
+        vec![ServerDiagnosticsGauge {
+            name: "core.threads.live".to_string(),
+            value: 1,
+        }]
+    );
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn server_diagnostics_requires_experimental_capability() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let mut app_server = TestAppServer::builder()
+        .with_codex_home(codex_home.path())
+        .build()
+        .await?;
+    let initialization = app_server
+        .initialize_with_capabilities(
+            ClientInfo {
+                name: DEFAULT_CLIENT_NAME.to_string(),
+                title: None,
+                version: "0.1.0".to_string(),
+            },
+            Some(InitializeCapabilities {
+                experimental_api: false,
+                ..Default::default()
+            }),
+        )
+        .await?;
+    assert!(matches!(initialization, JSONRPCMessage::Response(_)));
+
+    let request_id = app_server
+        .send_raw_request("server/diagnostics", Some(json!({})))
+        .await?;
+    let error = timeout(
+        READ_TIMEOUT,
+        app_server.read_stream_until_error_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+
+    assert_eq!(error.error.code, -32600);
+    assert_eq!(
+        error.error.message,
+        "server/diagnostics requires experimentalApi capability"
+    );
+    assert_eq!(error.error.data, None);
+
+    Ok(())
+}