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/mcp-server/src/message_processor.rs

@@ -1,4 +1,5 @@
 use std::collections::HashMap;
+use std::sync::Arc;
 
 use codex_arg0::Arg0DispatchPaths;
 use codex_core::AuthManager;
@@ -7,6 +8,7 @@ use codex_core::config::Config;
 use codex_core::default_client::USER_AGENT_SUFFIX;
 use codex_core::default_client::get_codex_user_agent;
 use codex_core::models_manager::collaboration_mode_presets::CollaborationModesConfig;
+use codex_exec_server::EnvironmentManager;
 use codex_features::Feature;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionSource;
@@ -27,7 +29,6 @@ use rmcp::model::RequestId;
 use rmcp::model::ServerCapabilities;
 use rmcp::model::ToolsCapability;
 use serde_json::json;
-use std::sync::Arc;
 use tokio::sync::Mutex;
 use tokio::task;
 
@@ -52,6 +53,7 @@ impl MessageProcessor {
         outgoing: OutgoingMessageSender,
         arg0_paths: Arg0DispatchPaths,
         config: Arc<Config>,
+        environment_manager: Arc<EnvironmentManager>,
     ) -> Self {
         let outgoing = Arc::new(outgoing);
         let auth_manager = AuthManager::shared(
@@ -68,6 +70,7 @@ impl MessageProcessor {
                     .features
                     .enabled(Feature::DefaultModeRequestUserInput),
             },
+            environment_manager,
         ));
         Self {
             outgoing,