fix: Ignore protected headers in outer message part (#6357)

Delta Chat always adds protected headers to the inner encrypted or signed message, so if a protected
header is only present in the outer part, it should be ignored because it's probably added by the
server or somebody else. The exceptions are Subject and List-ID because there are known cases when
they are only present in the outer message part.

Also treat any Chat-* headers as protected. This fixes e.g. a case when the server injects a
"Chat-Version" IMF header tricking Delta Chat into thinking that it's a chat message.

Also handle "Auto-Submitted" and "Autocrypt-Setup-Message" as protected headers on the receiver
side, this was apparently forgotten.

src/decrypt.rs

@@ -178,7 +178,8 @@ mod tests {
         let bob = TestContext::new_bob().await;
         receive_imf(&bob, attachment_mime, false).await?;
         let msg = bob.get_last_msg().await;
-        assert_eq!(msg.text, "Hello from Thunderbird!");
+        // Subject should be prepended because the attachment doesn't have "Chat-Version".
+        assert_eq!(msg.text, "Hello, Bob! – Hello from Thunderbird!");
 
         Ok(())
     }