feat: mimeparser: Omit Legacy Display Elements (#7130)

Omit Legacy Display Elements from "text/plain" and "text/html" (implement 4.5.3.{2,3} of
https://www.rfc-editor.org/rfc/rfc9788 "Header Protection for Cryptographically Protected Email").

src/mimeparser/mimeparser_tests.rs

@@ -1751,6 +1751,39 @@ async fn test_time_in_future() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_hp_legacy_display() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let bob = &tcm.bob().await;
+
+    let mut msg = Message::new_text(
+        "Subject: Dinner plans\n\
+        \n\
+        Let's eat"
+            .to_string(),
+    );
+    msg.set_subject("Dinner plans".to_string());
+    let chat_id = alice.create_chat(bob).await.id;
+    alice.set_config_bool(Config::TestHooks, true).await?;
+    *alice.pre_encrypt_mime_hook.lock() = Some(|_, mut mime| {
+        for (h, v) in &mut mime.headers {
+            if h == "Content-Type" {
+                if let mail_builder::headers::HeaderType::ContentType(ct) = v {
+                    *ct = ct.clone().attribute("hp-legacy-display", "1");
+                }
+            }
+        }
+        mime
+    });
+    let sent_msg = alice.send_msg(chat_id, &mut msg).await;
+
+    let msg_bob = bob.recv_msg(&sent_msg).await;
+    assert_eq!(msg_bob.subject, "Dinner plans");
+    assert_eq!(msg_bob.text, "Let's eat");
+    Ok(())
+}
+
 /// Tests that subject is not prepended to the message
 /// when bot receives it.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]