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/tests/suite/client.rs

@@ -829,6 +829,9 @@ async fn prefers_apikey_when_config_prefers_apikey_even_with_chatgpt_tokens() {
                 .features
                 .enabled(Feature::DefaultModeRequestUserInput),
         },
+        Arc::new(codex_exec_server::EnvironmentManager::new(
+            /*exec_server_url*/ None,
+        )),
     );
     let NewThread { thread: codex, .. } = thread_manager
         .start_thread(config)