Skip nested tool call parallel test on Windows (#14505)

**Summary**
- disable the `code_mode_nested_tool_calls_can_run_in_parallel` test on
Windows where `exec_command` is unavailable

**Testing**
- Not run (not requested)

codex-rs/core/tests/suite/code_mode.rs

@@ -233,6 +233,7 @@ add_content(JSON.stringify(await exec_command({ cmd: "printf code_mode_exec_mark
     Ok(())
 }
 
+#[cfg_attr(windows, ignore = "flaky on windows")]
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn code_mode_nested_tool_calls_can_run_in_parallel() -> Result<()> {
     skip_if_no_network!(Ok(()));