`MessageIDHeader` does not trim whitespace

Bug report

Bug description:

I'm seeing many emails with headers broken across multiple lines. When this happens to Message-Id, the parsed header value will include a leading space:

eml = dedent("""\
    From: "Foo Bar" <foo@bar.com>
    To: Baz Baq <baz@baq.com>
    Date: Thu, 22 May 2025 16:15:44 +0000
    Message-ID:
     <BY5PR09MB5490557EF2B28EC8E707108FBF99A@BY5PR09MB5490.namprd09.prod.outlook.com>
    Content-Type: text/plain

    Hello
""")
msg = Parser(policy=default_policy).parsestr(eml)
print(f"|{msg['Message-ID']}|") # prints | <BY5PR09MB5490557EF2B28EC8E707108FBF99A@BY5PR09MB5490.namprd09.prod.outlook.com>|

CPython versions tested on:

3.11

Operating systems tested on:

Linux

在 CPython 的电子邮件解析代码中搜索 MessageIDHeader，并跟踪 Parser 在使用 default_policy 时如何处理报告中所示的折叠 Message-ID。为该示例添加回归测试覆盖，并验证解析出的值不包含前导空白。