Improve ``NameError`` message when non-mangled name exists in the global scope

Bug report

Bug description:

Here is the code:

def __a():
    print("A")


class A:
    def b(self):
        __a()


a = A()
a.b()

It was expected to show a short message "A". However, I got a NameError:

❯ python3.12 bug\ represent.py
Traceback (most recent call last):
  File "/home/PC-Killer/python_work/Demos/bug represent.py", line 11, in <module>
    a.b()
  File "/home/PC-Killer/python_work/Demos/bug represent.py", line 7, in b
    __a()
    ^^^
NameError: name '_A__a' is not defined
❯ python3.13 bug\ represent.py
Traceback (most recent call last):
  File "/home/PC-Killer/python_work/Demos/bug represent.py", line 11, in <module>
    a.b()
    ~~~^^
  File "/home/PC-Killer/python_work/Demos/bug represent.py", line 7, in b
    __a()
    ^^^
NameError: name '_A__a' is not defined

It seems that the name had been translated in an unexpected way.

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

Linux

使用报告的 Python 版本运行提供的 bug represent.py 复现程序。没有指定 CPython 源文件或测试，因此请定位现有的名称改编和 NameError 建议测试，并跟踪它们的入口点。完成的标准是：示例生成预期的简洁消息，同时不回归相关的 NameError 行为。