Fix Windows exec-server output test flake (#17409)

Problem: The Windows exec-server test command could let separator
whitespace become part of `echo` output, making the exact
retained-output assertion flaky.

Solution: Tighten the Windows `cmd.exe` command by placing command
separators directly after the echoed tokens so stdout remains
deterministic while preserving the exact assertion.

codex-rs/exec-server/src/server/handler/tests.rs

@@ -275,7 +275,7 @@ async fn output_and_exit_are_retained_after_notification_receiver_closes() {
             process_id.as_str(),
             shell_argv(
                 "sleep 0.05; printf 'first\\n'; sleep 0.05; printf 'second\\n'",
-                "echo first && ping -n 2 127.0.0.1 >NUL && echo second",
+                "echo first&& ping -n 2 127.0.0.1 >NUL&& echo second",
             ),
         ))
         .await