configparser.write() loses UNNAMED_SECTION items to DEFAULT on round-trip

Bug description

ConfigParser.write() outputs UNNAMED_SECTION items after the [DEFAULT] header. When the output is read back, the headerless items appear under [DEFAULT] and are lost from UNNAMED_SECTION.

import configparser, io

US = configparser.UNNAMED_SECTION
cfg = configparser.ConfigParser(allow_unnamed_section=True)
cfg.read_string('key1 = val1\n[DEFAULT]\ndkey = dval\n[sect1]\nkey2 = val2\n')

out = io.StringIO()
cfg.write(out)

cfg2 = configparser.ConfigParser(allow_unnamed_section=True)
cfg2.read_string(out.getvalue())
print('key1' in cfg2[US])       # False -- lost
print('key1' in cfg2.defaults())  # True -- migrated to DEFAULT

CPython versions tested on

3.14, main

Operating systems tested on

macOS

Linked PRs

• gh-154597

重现 issue 中展示的往返过程，然后检查 ConfigParser.write() 和 read_string()，跟踪 UNNAMED_SECTION 中的项目是如何输出和解析的。完成的标准是：写入并读取配置后，不带标头的键仍保留在 cfg2[UNNAMED_SECTION] 中；关联的 PR gh-154597 表明相关工作已经在进行中。