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/core/tests/suite/client.rs

@@ -1103,9 +1103,7 @@ 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,
-        )),
+        Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
         /*analytics_events_client*/ None,
     );
     let NewThread { thread: codex, .. } = thread_manager