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/exec/src/lib.rs

@@ -502,9 +502,9 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
         cloud_requirements: run_cloud_requirements,
         feedback: CodexFeedback::new(),
         log_db: None,
-        environment_manager: std::sync::Arc::new(EnvironmentManager::new(
-            EnvironmentManagerArgs::from_env(local_runtime_paths),
-        )),
+        environment_manager: std::sync::Arc::new(
+            EnvironmentManager::new(EnvironmentManagerArgs::new(local_runtime_paths)).await,
+        ),
         config_warnings,
         session_source: SessionSource::Exec,
         enable_codex_api_key_env: true,