email.message generates invalid encoded header

Bug report

Bug description:

A message header generated by email.message.Message can contain invalid RFC 2047 encoded-words.

import sys
from email.message import Message
from email.policy import SMTP

msg = Message(policy=SMTP)
msg["Subject"] = "Re: some few filler words here RE: Routeraustausch und übriggebliebene Glasfaser"
sys.stdout.buffer.write(msg.as_bytes())

Result:

Subject: Re: some few filler words here RE: Routeraustausch und =?utf-8?q??=
 =?utf-8?q?=C3=BCbriggebliebene?= Glasfaser

The encoded-word in the first line is invalid. Per RFC 2047 the "encoded-text" must not be empty:

encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
encoded-text = 1*<Any printable ASCII character other than "?"
                 or SPACE>

The error does not appear in this case with the default policy because the header value is split up entirely differently.

CPython versions tested on:

3.13, 3.12

Operating systems tested on:

Windows, Other

Linked PRs

• gh-136098
• gh-136099

Comece reproduzindo o exemplo usando email.message.Message com a SMTP policy e inspecione o header folding e o caminho de codificação RFC 2047. Considera-se concluído quando o Subject gerado não contiver nenhum encoded-text vazio e permanecer RFC 2047-valid, com cobertura de regressão para esta entrada; o relatório lista os PRs vinculados gh-136098 e gh-136099.