Preserve Windows proxy settings in Guardian sessions (#40641)

## Why

Guardian sessions can run alongside another Windows sandbox launch and should not reconcile the persistent proxy settings established by that launch.

## What changed

- Start internal Guardian sessions with `WindowsSandboxProxySettingsMode::Preserve`.
- Continue using `Reconcile` for other sessions.

## Testing

Added a thread manager test that verifies a parent session reconciles proxy settings while its internal Guardian session preserves them.

GitOrigin-RevId: c6a31637ecdf828548cfd7153da7e9405abe34fa

codex-rs/core/src/thread_manager.rs

@@ -55,6 +55,7 @@ use codex_protocol::mcp::OPENAI_STANDARD_FORM_INPUT_EXTENSION_ID;
 use codex_protocol::openai_models::ModelPreset;
 use codex_protocol::protocol::Event;
 use codex_protocol::protocol::EventMsg;
+use codex_protocol::protocol::InternalSessionSource;
 use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::SessionConfiguredEvent;
@@ -1938,6 +1939,14 @@ impl ThreadManagerState {
             starting.retain(|runtime| runtime.strong_count() != 0);
             starting.push(Arc::downgrade(&source_changed_during_startup));
         }
+        let windows_sandbox_proxy_settings_mode = if matches!(
+            &session_source,
+            SessionSource::Internal(InternalSessionSource::Guardian)
+        ) {
+            codex_sandboxing::WindowsSandboxProxySettingsMode::Preserve
+        } else {
+            codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile
+        };
         let (session, io) = Box::pin(Session::spawn(SessionSpawnArgs {
             config,
             allow_provider_model_fallback,
@@ -1977,8 +1986,7 @@ impl ThreadManagerState {
             external_time_provider: self.external_time_provider.clone(),
             inherited_multi_agent_version: multi_agent_version,
             git_enrichment_policy: GitEnrichmentPolicy::Fresh,
-            windows_sandbox_proxy_settings_mode:
-                codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
+            windows_sandbox_proxy_settings_mode,
         }))
         .await?;
         // Enable Full Access form input only after session startup so a required MCP server cannot

codex-rs/core/src/thread_manager_tests.rs

@@ -878,6 +878,51 @@ async fn start_thread_keeps_internal_threads_hidden_from_normal_lookups() {
     assert!(manager.list_thread_ids().await.is_empty());
 }
 
+#[tokio::test]
+async fn spawn_internal_guardian_session_preserves_windows_sandbox_proxy_settings() {
+    let temp_dir = tempdir().expect("tempdir");
+    let mut config = test_config().await;
+    config.codex_home = temp_dir.path().join("codex-home").abs();
+    config.cwd = config.codex_home.abs();
+    std::fs::create_dir_all(&config.codex_home).expect("create codex home");
+
+    let manager = ThreadManager::with_models_provider_and_home_for_tests(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+        config.codex_home.to_path_buf(),
+        Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
+    );
+    let parent = manager
+        .start_thread(StartThreadOptions::new(config.clone()))
+        .await
+        .expect("start parent thread");
+    let reviewer = manager
+        .spawn_internal_session(
+            parent.thread_id,
+            StartThreadOptions {
+                session_source: Some(SessionSource::Internal(InternalSessionSource::Guardian)),
+                ..StartThreadOptions::new(config)
+            },
+        )
+        .await
+        .expect("start internal reviewer");
+
+    assert_eq!(
+        (
+            parent.thread.session.windows_sandbox_proxy_settings_mode,
+            reviewer.thread.session.windows_sandbox_proxy_settings_mode,
+        ),
+        (
+            codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile,
+            codex_sandboxing::WindowsSandboxProxySettingsMode::Preserve,
+        )
+    );
+
+    manager
+        .shutdown_all_threads_bounded(Duration::from_secs(10))
+        .await;
+}
+
 #[tokio::test]
 async fn spawn_internal_session_preserves_parent_lineage_without_forking_history() {
     struct ParentLifecycleContributor {