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/tests/common/test_codex.rs

@@ -384,15 +384,17 @@ impl TestCodexBuilder {
             .exec_server_url
             .clone()
             .or_else(|| test_env.exec_server_url().map(str::to_owned));
-        let environment_manager = Arc::new(codex_exec_server::EnvironmentManager::new(
-            codex_exec_server::EnvironmentManagerArgs {
+        let local_runtime_paths = codex_exec_server::ExecServerRuntimePaths::new(
+            std::env::current_exe()?,
+            /*codex_linux_sandbox_exe*/ None,
+        )?;
+        let environment_manager = Arc::new(
+            codex_exec_server::EnvironmentManager::create_for_tests(
                 exec_server_url,
-                local_runtime_paths: codex_exec_server::ExecServerRuntimePaths::new(
-                    std::env::current_exe()?,
-                    /*codex_linux_sandbox_exe*/ None,
-                )?,
-            },
-        ));
+                local_runtime_paths,
+            )
+            .await,
+        );
         let file_system = test_env.environment().get_filesystem();
         let mut workspace_setups = vec![];
         swap(&mut self.workspace_setups, &mut workspace_setups);