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/mcp-server/src/lib.rs

@@ -60,12 +60,15 @@ pub async fn run_main(
     arg0_paths: Arg0DispatchPaths,
     cli_config_overrides: CliConfigOverrides,
 ) -> IoResult<()> {
-    let environment_manager = Arc::new(EnvironmentManager::new(EnvironmentManagerArgs::from_env(
-        ExecServerRuntimePaths::from_optional_paths(
-            arg0_paths.codex_self_exe.clone(),
-            arg0_paths.codex_linux_sandbox_exe.clone(),
-        )?,
-    )));
+    let environment_manager = Arc::new(
+        EnvironmentManager::new(EnvironmentManagerArgs::new(
+            ExecServerRuntimePaths::from_optional_paths(
+                arg0_paths.codex_self_exe.clone(),
+                arg0_paths.codex_linux_sandbox_exe.clone(),
+            )?,
+        ))
+        .await,
+    );
     // Parse CLI overrides once and derive the base Config eagerly so later
     // components do not need to work with raw TOML values.
     let cli_kv_overrides = cli_config_overrides.parse_overrides().map_err(|e| {