Add environment provider snapshot (#20058)

## Summary
- Change `EnvironmentProvider` to return concrete `Environment`
instances instead of `EnvironmentConfigurations`.
- Make `DefaultEnvironmentProvider` provide the provider-visible `local`
environment plus optional `remote` environment from
`CODEX_EXEC_SERVER_URL`.
- Keep `EnvironmentManager` as the concrete cache while exposing its own
explicit local environment for `local_environment()` fallback paths.

## Validation
- `just fmt`
- `git diff --check`

---------

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

codex-rs/core/src/connectors.rs

@@ -201,7 +201,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_options_and_status(
         config.codex_linux_sandbox_exe.clone(),
     )?;
     let environment_manager =
-        EnvironmentManager::new(EnvironmentManagerArgs::from_env(local_runtime_paths));
+        EnvironmentManager::new(EnvironmentManagerArgs::new(local_runtime_paths)).await;
     list_accessible_connectors_from_mcp_tools_with_environment_manager(
         config,
         force_refetch,