`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 的電子郵件解析程式碼中搜尋 MessageIDHeader，並追蹤 Parser 使用 default_policy 時如何處理報告中所示的摺疊 Message-ID。為該範例新增回歸測試涵蓋，並驗證解析出的值不包含前導空白。