app-server: prepare to run initialized rpcs concurrently (#17372)

## Summary

- Refactors `MessageProcessor` and per-connection session state so
initialized service RPC handling can be moved into spawned tasks in a
follow-up PR.
- Shares the processor and initialized session data with
`Arc`/`OnceLock` instead of mutable borrowed connection state.
- Keeps initialized request handling synchronous in this PR; it does
**not** call `tokio::spawn` for service RPCs yet.

## Testing

- `just fmt`
- `cargo test -p codex-app-server` *(fails on existing hardening gaps
covered by #17375, #17376, and #17377; the pipelined config regression
passed before the unrelated failures)*
- `just fix -p codex-app-server`

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

@@ -109,9 +109,9 @@ fn tracing_test_guard() -> &'static tokio::sync::Mutex<()> {
 struct TracingHarness {
     _server: MockServer,
     _codex_home: TempDir,
-    processor: MessageProcessor,
+    processor: Arc<MessageProcessor>,
     outgoing_rx: mpsc::Receiver<crate::outgoing_message::OutgoingEnvelope>,
-    session: ConnectionSessionState,
+    session: Arc<ConnectionSessionState>,
     tracing: &'static TestTracing,
 }
 
@@ -129,7 +129,7 @@ impl TracingHarness {
             _codex_home: codex_home,
             processor,
             outgoing_rx,
-            session: ConnectionSessionState::default(),
+            session: Arc::new(ConnectionSessionState::default()),
             tracing,
         };
 
@@ -152,7 +152,7 @@ impl TracingHarness {
                 /*trace*/ None,
             )
             .await;
-        assert!(harness.session.initialized);
+        assert!(harness.session.initialized());
 
         Ok(harness)
     }
@@ -182,7 +182,7 @@ impl TracingHarness {
                 TEST_CONNECTION_ID,
                 request,
                 AppServerTransport::Stdio,
-                &mut self.session,
+                Arc::clone(&self.session),
             )
             .await;
         read_response(&mut self.outgoing_rx, request_id).await
@@ -230,14 +230,14 @@ async fn build_test_config(codex_home: &Path, server_uri: &str) -> Result<Config
 fn build_test_processor(
     config: Arc<Config>,
 ) -> (
-    MessageProcessor,
+    Arc<MessageProcessor>,
     mpsc::Receiver<crate::outgoing_message::OutgoingEnvelope>,
 ) {
     let (outgoing_tx, outgoing_rx) = mpsc::channel(16);
     let outgoing = Arc::new(OutgoingMessageSender::new(outgoing_tx));
     let auth_manager =
         AuthManager::shared_from_config(config.as_ref(), /*enable_codex_api_key_env*/ false);
-    let processor = MessageProcessor::new(MessageProcessorArgs {
+    let processor = Arc::new(MessageProcessor::new(MessageProcessorArgs {
         outgoing,
         arg0_paths: Arg0DispatchPaths::default(),
         config,
@@ -252,7 +252,7 @@ fn build_test_processor(
         auth_manager,
         rpc_transport: AppServerRpcTransport::Stdio,
         remote_control_handle: None,
-    });
+    }));
     (processor, outgoing_rx)
 }