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/app-server/tests/suite/v2/mcp_resource.rs

@@ -204,7 +204,7 @@ async fn mcp_resource_read_returns_error_for_unknown_thread() -> Result<()> {
         thread_config_loader: Arc::new(codex_config::NoopThreadConfigLoader),
         feedback: CodexFeedback::new(),
         log_db: None,
-        environment_manager: Arc::new(EnvironmentManager::new(/*exec_server_url*/ None)),
+        environment_manager: Arc::new(EnvironmentManager::default_for_tests()),
         config_warnings: Vec::new(),
         session_source: SessionSource::Cli,
         enable_codex_api_key_env: false,