Add cached environment manager for exec server URL (#15785)

Add environment manager that is a singleton and is created early in
app-server (before skill manager, before config loading).

Use an environment variable to point to a running exec server.

codex-rs/core/src/thread_manager_tests.rs

@@ -244,6 +244,9 @@ async fn shutdown_all_threads_bounded_submits_shutdown_to_every_thread() {
         CodexAuth::from_api_key("dummy"),
         config.model_provider.clone(),
         config.codex_home.clone(),
+        Arc::new(codex_exec_server::EnvironmentManager::new(
+            /*exec_server_url*/ None,
+        )),
     );
     let thread_1 = manager
         .start_thread(config.clone())
@@ -292,6 +295,9 @@ async fn new_uses_configured_openai_provider_for_model_refresh() {
         auth_manager,
         SessionSource::Exec,
         CollaborationModesConfig::default(),
+        Arc::new(codex_exec_server::EnvironmentManager::new(
+            /*exec_server_url*/ None,
+        )),
     );
 
     let _ = manager.list_models(RefreshStrategy::Online).await;
@@ -419,6 +425,9 @@ async fn interrupted_fork_snapshot_does_not_synthesize_turn_id_for_legacy_histor
         auth_manager.clone(),
         SessionSource::Exec,
         CollaborationModesConfig::default(),
+        Arc::new(codex_exec_server::EnvironmentManager::new(
+            /*exec_server_url*/ None,
+        )),
     );
 
     let source = manager
@@ -516,6 +525,9 @@ async fn interrupted_fork_snapshot_preserves_explicit_turn_id() {
         auth_manager.clone(),
         SessionSource::Exec,
         CollaborationModesConfig::default(),
+        Arc::new(codex_exec_server::EnvironmentManager::new(
+            /*exec_server_url*/ None,
+        )),
     );
 
     let source = manager
@@ -602,6 +614,9 @@ async fn interrupted_fork_snapshot_uses_persisted_mid_turn_history_without_live_
         auth_manager.clone(),
         SessionSource::Exec,
         CollaborationModesConfig::default(),
+        Arc::new(codex_exec_server::EnvironmentManager::new(
+            /*exec_server_url*/ None,
+        )),
     );
 
     let source = manager