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/memories/tests.rs

@@ -484,6 +484,9 @@ mod phase2 {
                 CodexAuth::from_api_key("dummy"),
                 config.model_provider.clone(),
                 config.codex_home.clone(),
+                std::sync::Arc::new(codex_exec_server::EnvironmentManager::new(
+                    /*exec_server_url*/ None,
+                )),
             );
             let (mut session, _turn_context) = make_session_and_context().await;
             session.services.state_db = Some(Arc::clone(&state_db));