[codex] request desktop attestation from app (#20619)

## Summary

TL;DR: teaches `codex-rs` / app-server to request a desktop-provided
attestation token and attach it as `x-oai-attestation` on the scoped
ChatGPT Codex request paths.

![DeviceCheck attestation
interface](https://raw.githubusercontent.com/openai/codex/dev/jm/devicecheck-diagram-assets/pr-assets/devicecheck-attestation-interface.png)

## Details

This PR teaches the Codex app-server runtime how to request and attach
an attestation token. It does not generate DeviceCheck tokens directly;
instead, it relies on the connected desktop app to advertise that it can
generate attestation and then asks that app for a fresh header value
when needed.

The flow is:

1. The Codex desktop app connects to app-server.
2. During `initialize`, the app can advertise that it supports
`requestAttestation`.
3. Before app-server calls selected ChatGPT Codex endpoints, it sends
the internal server request `attestation/generate` to the app.
4. app-server receives a pre-encoded header value back.
5. app-server forwards that value as `x-oai-attestation` on the scoped
outbound requests.

The code in this repo is mostly protocol and runtime plumbing: it adds
the app-server request/response shape, introduces an attestation
provider in core, wires that provider into Responses / compaction /
realtime setup paths, and covers the intended scoping with tests. The
signed macOS DeviceCheck generation remains owned by the desktop app PR.

## Related PR

- Codex desktop app implementation:
https://github.com/openai/openai/pull/878649

## Validation

<details>
<summary>Tests run</summary>

```sh
cargo test -p codex-app-server-protocol
cargo test -p codex-core attestation --lib
cargo test -p codex-app-server --lib attestation
```

Also ran:

```sh
just fix -p codex-core
just fix -p codex-app-server
just fix -p codex-app-server-protocol
just fmt
just write-app-server-schema
```

</details>

<details>
<summary>E2E DeviceCheck validation</summary>

First validated the signed desktop app boundary directly: launched a
packaged signed `Codex.app`, sent `attestation/generate`, decoded the
returned `v1.` attestation header, and validated the extracted
DeviceCheck token with `personal/jm/verify_devicecheck_token.py` using
bundle ID `com.openai.codex`. Apple returned `status_code: 200` and
`is_ok: true`.

Then ran the fuller app + app-server flow. The packaged `Codex.app`
launched a current-branch app-server via `CODEX_CLI_PATH`, and a local
MITM proxy intercepted outbound `chatgpt.com` traffic. The app-server
requested `attestation/generate` from the real Electron app process, and
the intercepted `/backend-api/codex/responses` traffic included
`x-oai-attestation` on both routes:

```text
GET  /backend-api/codex/responses  Upgrade: websocket  x-oai-attestation: present
POST /backend-api/codex/responses  Upgrade: none       x-oai-attestation: present
```

The captured header decoded to a DeviceCheck token that also validated
with Apple for `com.openai.codex` (`status_code: 200`, `is_ok: true`,
team `2DC432GLL2`).

</details>

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/src/message_processor.rs

@@ -4,6 +4,7 @@ use std::sync::Arc;
 use std::sync::OnceLock;
 use std::sync::atomic::AtomicBool;
 
+use crate::attestation::app_server_attestation_provider;
 use crate::config_manager::ConfigManager;
 use crate::connection_rpc_gate::ConnectionRpcGate;
 use crate::error_code::invalid_request;
@@ -34,6 +35,7 @@ use crate::request_processors::WindowsSandboxRequestProcessor;
 use crate::request_serialization::QueuedInitializedRequest;
 use crate::request_serialization::RequestSerializationQueueKey;
 use crate::request_serialization::RequestSerializationQueues;
+use crate::thread_state::ConnectionCapabilities;
 use crate::thread_state::ThreadStateManager;
 use crate::transport::AppServerTransport;
 use crate::transport::RemoteControlHandle;
@@ -82,6 +84,7 @@ use tokio::time::timeout;
 use tracing::Instrument;
 
 const EXTERNAL_AUTH_REFRESH_TIMEOUT: Duration = Duration::from_secs(10);
+
 #[derive(Clone)]
 struct ExternalAuthRefreshBridge {
     outgoing: Arc<OutgoingMessageSender>,
@@ -186,6 +189,7 @@ pub(crate) struct InitializedConnectionSessionState {
     pub(crate) opted_out_notification_methods: HashSet<String>,
     pub(crate) app_server_client_name: String,
     pub(crate) client_version: String,
+    pub(crate) request_attestation: bool,
 }
 
 impl Default for ConnectionSessionState {
@@ -231,6 +235,12 @@ impl ConnectionSessionState {
             .map(|session| session.client_version.as_str())
     }
 
+    pub(crate) fn request_attestation(&self) -> bool {
+        self.initialized
+            .get()
+            .is_some_and(|session| session.request_attestation)
+    }
+
     pub(crate) fn initialize(&self, session: InitializedConnectionSessionState) -> Result<(), ()> {
         self.initialized.set(session).map_err(|_| ())
     }
@@ -280,6 +290,7 @@ impl MessageProcessor {
         auth_manager.set_external_auth(Arc::new(ExternalAuthRefreshBridge {
             outgoing: outgoing.clone(),
         }));
+        let thread_state_manager = ThreadStateManager::new();
         // The thread store is intentionally process-scoped. Config reloads can
         // affect per-thread behavior, but they must not move newly started,
         // resumed, or forked threads to a different persistence backend/root.
@@ -293,13 +304,16 @@ impl MessageProcessor {
             Arc::clone(&thread_store),
             state_db.clone(),
             installation_id,
+            Some(app_server_attestation_provider(
+                outgoing.clone(),
+                thread_state_manager.clone(),
+            )),
         ));
         thread_manager
             .plugins_manager()
             .set_analytics_events_client(analytics_events_client.clone());
 
         let pending_thread_unloads = Arc::new(Mutex::new(HashSet::new()));
-        let thread_state_manager = ThreadStateManager::new();
         let thread_watch_manager =
             crate::thread_status::ThreadWatchManager::new_with_outgoing(outgoing.clone());
         let thread_list_state_permit = Arc::new(Semaphore::new(/*permits*/ 1));
@@ -620,9 +634,18 @@ impl MessageProcessor {
             .await;
     }
 
-    pub(crate) async fn connection_initialized(&self, connection_id: ConnectionId) {
+    pub(crate) async fn connection_initialized(
+        &self,
+        connection_id: ConnectionId,
+        request_attestation: bool,
+    ) {
         self.thread_processor
-            .connection_initialized(connection_id)
+            .connection_initialized(
+                connection_id,
+                ConnectionCapabilities {
+                    request_attestation,
+                },
+            )
             .await;
     }
 
@@ -718,7 +741,12 @@ impl MessageProcessor {
                 .await?;
             if connection_initialized {
                 self.thread_processor
-                    .connection_initialized(connection_id)
+                    .connection_initialized(
+                        connection_id,
+                        ConnectionCapabilities {
+                            request_attestation: session.request_attestation(),
+                        },
+                    )
                     .await;
             }
             return Ok(());