app-server: run initialized rpcs with keyed serialization (#17373)

## Why

Initialized app-server RPCs no longer need to bottleneck behind one
request processor path. Running them concurrently improves
responsiveness, but several request families still mutate shared state
or depend on ordered side effects. Those stateful families need an
auditable serialization contract so concurrency does not reorder thread,
config, auth, command, watcher, MCP, or similar state transitions.

This PR keeps that boundary explicit: stateful work is serialized by the
smallest useful key, while intentionally read-only or externally
concurrent work remains unkeyed. In particular, `thread/list` and
`thread/turns/list` explicitly have no serialization because they
primarily read append-only rollout storage and should continue to be
served concurrently.

## What changed

- Adds `ClientRequest::serialization_scope()` in `app-server-protocol`
and requires every client request definition to declare its
serialization behavior.
- Introduces keyed request scopes for thread, thread path, command exec
process, fuzzy search session, fs watch, MCP OAuth, and global state
buckets such as config, account auth, memory, and device keys.
- Routes initialized app-server RPCs through per-key FIFO serialization
while allowing unkeyed initialized requests to run concurrently.
- Cancels in-flight initialized RPC work when the connection disconnects
or the app-server exits so spawned request tasks do not outlive their
session.
- Adds focused coverage for representative keyed and unkeyed
serialization scopes, including explicitly concurrent
`thread/turns/list` behavior.

## Validation

- Added protocol tests for representative keyed serialization scopes and
intentionally unkeyed request families.
- Added app-server request serialization tests covering per-key FIFO
behavior, concurrent unkeyed execution, disconnect shutdown, and config
read-after-write ordering.
- Local focused protocol validation after the latest rebase is currently
blocked by packageproxy failing to resolve locked `rustls-webpki
0.103.13`; CI is expected to provide the full validation signal.

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

@@ -9,6 +9,7 @@ use crate::codex_message_processor::CodexMessageProcessor;
 use crate::codex_message_processor::CodexMessageProcessorArgs;
 use crate::config_api::ConfigApi;
 use crate::config_manager::ConfigManager;
+use crate::connection_rpc_gate::ConnectionRpcGate;
 use crate::device_key_api::DeviceKeyApi;
 use crate::error_code::invalid_request;
 use crate::external_agent_config_api::ExternalAgentConfigApi;
@@ -18,6 +19,9 @@ use crate::outgoing_message::ConnectionId;
 use crate::outgoing_message::ConnectionRequestId;
 use crate::outgoing_message::OutgoingMessageSender;
 use crate::outgoing_message::RequestContext;
+use crate::request_serialization::QueuedInitializedRequest;
+use crate::request_serialization::RequestSerializationQueueKey;
+use crate::request_serialization::RequestSerializationQueues;
 use crate::transport::AppServerTransport;
 use crate::transport::ConnectionOrigin;
 use crate::transport::RemoteControlHandle;
@@ -167,11 +171,13 @@ pub(crate) struct MessageProcessor {
     config_warnings: Arc<Vec<ConfigWarningNotification>>,
     rpc_transport: AppServerRpcTransport,
     remote_control_handle: Option<RemoteControlHandle>,
+    request_serialization_queues: RequestSerializationQueues,
 }
 
 #[derive(Debug)]
 pub(crate) struct ConnectionSessionState {
     origin: ConnectionOrigin,
+    pub(crate) rpc_gate: Arc<ConnectionRpcGate>,
     initialized: OnceLock<InitializedConnectionSessionState>,
 }
 
@@ -193,6 +199,7 @@ impl ConnectionSessionState {
     pub(crate) fn new(origin: ConnectionOrigin) -> Self {
         Self {
             origin,
+            rpc_gate: Arc::new(ConnectionRpcGate::new()),
             initialized: OnceLock::new(),
         }
     }
@@ -344,6 +351,7 @@ impl MessageProcessor {
             config_warnings: Arc::new(config_warnings),
             rpc_transport,
             remote_control_handle,
+            request_serialization_queues: RequestSerializationQueues::default(),
         }
     }
 
@@ -540,7 +548,12 @@ impl MessageProcessor {
         self.codex_message_processor.shutdown_threads().await;
     }
 
-    pub(crate) async fn connection_closed(&self, connection_id: ConnectionId) {
+    pub(crate) async fn connection_closed(
+        &self,
+        connection_id: ConnectionId,
+        session_state: &ConnectionSessionState,
+    ) {
+        session_state.rpc_gate.shutdown().await;
         self.outgoing.connection_closed(connection_id).await;
         self.fs_watch_manager.connection_closed(connection_id).await;
         self.codex_message_processor
@@ -724,19 +737,46 @@ impl MessageProcessor {
             );
         }
 
+        let serialization_scope = codex_request.serialization_scope();
         let app_server_client_name = session.app_server_client_name().map(str::to_string);
         let client_version = session.client_version().map(str::to_string);
         let device_key_requests_allowed = session.allows_device_key_requests();
-        Arc::clone(self)
-            .handle_initialized_client_request(
-                connection_request_id,
-                codex_request,
-                request_context,
-                app_server_client_name,
-                client_version,
-                device_key_requests_allowed,
-            )
-            .await
+        let error_request_id = connection_request_id.clone();
+        let rpc_gate = Arc::clone(&session.rpc_gate);
+        let processor = Arc::clone(self);
+        let span = request_context.span();
+        let request = QueuedInitializedRequest::new(
+            rpc_gate,
+            async move {
+                let processor_for_request = Arc::clone(&processor);
+                let result = processor_for_request
+                    .handle_initialized_client_request(
+                        connection_request_id,
+                        codex_request,
+                        request_context,
+                        app_server_client_name,
+                        client_version,
+                        device_key_requests_allowed,
+                    )
+                    .await;
+                if let Err(error) = result {
+                    processor.outgoing.send_error(error_request_id, error).await;
+                }
+            }
+            .instrument(span),
+        );
+
+        if let Some(scope) = serialization_scope {
+            let key = RequestSerializationQueueKey::from_scope(connection_id, scope);
+            self.request_serialization_queues
+                .enqueue(key, request)
+                .await;
+        } else {
+            tokio::spawn(async move {
+                request.run().await;
+            });
+        }
+        Ok(())
     }
 
     async fn handle_initialized_client_request(