Fix stale TestAppServer rename in plugin_list test

[bolinfest]

Why

#25701 renamed the app-server test harness to TestAppServer, but it raced with #25681, which added a new plugin_list test call site still using the old McpProcess name. Once both changes met on main, app-server test builds failed before running the suite because McpProcess no longer exists in that scope.

This PR fixes that CI break by updating the remaining stale call site to the renamed helper.

What Changed

• Replaced the McpProcess::new(...) use in codex-rs/app-server/tests/suite/v2/plugin_list.rs with TestAppServer::new(...).
• Renamed the local variable from mcp to app_server at the same call site to match the helper rename.

Relevant code:

codex/codex-rs/app-server/tests/suite/v2/plugin_list.rs

Lines 234 to 246 in aadd9c9

	let mut app_server = TestAppServer::new(codex_home.path()).await?;
	timeout(DEFAULT_TIMEOUT, app_server.initialize()).await??;
	let request_id = app_server
	.send_plugin_installed_request(PluginInstalledParams {
	cwds: None,
	install_suggestion_plugin_names: None,
	})
	.await?;
	let response: JSONRPCResponse = timeout(
	DEFAULT_TIMEOUT,
	app_server.read_stream_until_response_message(RequestId::Integer(request_id)),

Verification

Not run locally; this is a compile fix for the app-server test harness rename.