fix(3p/lisp/mime4cl): make MIME-BODY-STREAM always return characters

Because OPEN-DECODED-FILE-PORTION only knows about transfer encodings it
would only return a character stream for 7bit encoded bodies. This
causes inconsistent behavior where some bodies would return binary and
some character streams. To fix this, we specialize MIME-BODY-STREAM for
MIME-TEXT parts which may or may not be a good enough solution.

We may actually want to make MIME-BODY-STREAM binary always and let the
user handle decoding?! This may be a good idea to take care after yet
another stream machinery redesign.

Since the mime4cl test suite doesn't test MIME-BODY-STREAM (much), add a
message generated by notemap that hits this issue to the mblog golden
test suite.

Change-Id: Ie340c42ced6c693af9b3c84b177408d6b6d2c9c4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12913
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

third_party/lisp/mime4cl/test/samples/mail-note-from-notemap.msg

@@ -0,0 +1,13 @@
+From: <sterni>
+Date: Wed, 25 Dec 2024 23:54:39 +0100
+X-Universally-Unique-Identifier: d2ccdc74-830c-41ee-9d64-2221f1c35449
+X-Uniform-Type-Identifier: com.apple.mail-note
+X-Mailer: notemap
+MIME-Version: 1.0
+Content-Type: text/plain; charset="utf-8"
+Content-Transfer-Encoding: quoted-printable
+Subject: =?utf-8?Q?test.txt?=
+X-TUID: clxSN23/djqG
+
+gr=C3=BC=C3=9Fe from notemap
+