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

Comienza reproduciendo el ejemplo con email.message.Message usando la SMTP policy e inspecciona el header folding y la ruta de codificación RFC 2047. Se considera terminado cuando el Subject generado no contiene ningún encoded-text vacío y sigue siendo RFC 2047-valid, con cobertura de regresión para esta entrada; el informe enumera los PRs vinculados gh-136098 y gh-136099.