Support multiple managed environments (#18401)

## Summary
- refactor EnvironmentManager to own keyed environments with
default/local lookup helpers
- keep remote exec-server client creation lazy until exec/fs use
- preserve disabled agent environment access separately from internal
local environment access

## Validation
- not run (per Codex worktree instruction to avoid tests/builds unless
requested)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/tests/file_system.rs

@@ -46,7 +46,7 @@ struct FileSystemContext {
 async fn create_file_system_context(use_remote: bool) -> Result<FileSystemContext> {
     if use_remote {
         let server = exec_server().await?;
-        let environment = Environment::create(Some(server.websocket_url().to_string())).await?;
+        let environment = Environment::create_for_tests(Some(server.websocket_url().to_string()))?;
         Ok(FileSystemContext {
             file_system: environment.get_filesystem(),
             _helper_paths: None,
@@ -214,7 +214,7 @@ async fn sandboxed_file_system_helper_finds_bwrap_on_preserved_path() -> Result<
     let helper_path = std::env::join_paths(path_entries)?;
 
     let server = exec_server_with_env([("PATH", helper_path.as_os_str())]).await?;
-    let environment = Environment::create(Some(server.websocket_url().to_string())).await?;
+    let environment = Environment::create_for_tests(Some(server.websocket_url().to_string()))?;
     let file_system = environment.get_filesystem();
     let workspace = tmp.path().join("workspace");
     std::fs::create_dir_all(&workspace)?;