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/codex_tests_guardian.rs

@@ -12,6 +12,7 @@ use crate::sandboxing::SandboxPermissions;
 use crate::tools::context::FunctionToolOutput;
 use crate::turn_diff_tracker::TurnDiffTracker;
 use codex_app_server_protocol::ConfigLayerSource;
+use codex_exec_server::EnvironmentManager;
 use codex_execpolicy::Decision;
 use codex_execpolicy::Evaluation;
 use codex_execpolicy::RuleMatch;
@@ -437,6 +438,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         config,
         auth_manager,
         models_manager,
+        environment_manager: Arc::new(EnvironmentManager::new(/*exec_server_url*/ None)),
         skills_manager,
         plugins_manager,
         mcp_manager,