test: test reception of multipart/alternative with text/calendar

In messages with text/plain, text/html and text/calendar parts
within a multipart/alternative, text/calendar part is currently ignored,
text/plain is displayed and HTML is available via HTML API.

src/receive_imf/receive_imf_tests.rs

@@ -5557,3 +5557,29 @@ async fn test_lookup_key_contact_by_address_self() -> Result<()> {
     );
     Ok(())
 }
+
+/// Tests reception of multipart/alternative
+/// with three parts, one of which is a calendar.
+///
+/// MS Exchange produces multipart/alternative
+/// messages with three parts:
+/// `text/plain`, `text/html` and `text/calendar`.
+///
+/// We display `text/plain` part in this case.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_calendar_alternative() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let t = &tcm.alice().await;
+    let raw = include_bytes!("../../test-data/message/calendar-alternative.eml");
+    let msg = receive_imf(t, raw, false).await?.unwrap();
+    assert_eq!(msg.msg_ids.len(), 1);
+
+    let text_msg = Message::load_from_db(t, msg.msg_ids[0]).await?;
+    assert_eq!(text_msg.text, "Subject was here – Hello!");
+    assert_eq!(text_msg.viewtype, Viewtype::Text);
+    assert!(text_msg.has_html());
+    let html = text_msg.get_id().get_html(t).await.unwrap().unwrap();
+    assert_eq!(html, "<b>Hello!</b>");
+
+    Ok(())
+}

test-data/message/calendar-alternative.eml

@@ -0,0 +1,30 @@
+From: Bob <bob@example.net>
+To: Alice <alice@example.org>
+Subject: Subject was here
+Date: Mon, 11 Aug 2025 10:15:52 +0000
+Message-ID: 
+	<DU2PR10MB7741CB7551F025C98CE1B0C0EE28A@DU2PR10MB7741.EURPRD10.PROD.OUTLOOK.COM>
+Content-Type: multipart/alternative;
+	boundary="_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_"
+MIME-Version: 1.0
+
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_
+Content-Type: text/plain; charset="utf-8"
+
+Hello!
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_
+Content-Type: text/html; charset="utf-8"
+
+<b>Hello!</b>
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_
+Content-Type: text/calendar; charset="utf-8"; method=REQUEST
+
+BEGIN:VCALENDAR
+METHOD:REQUEST
+PRODID:Microsoft Exchange Server 2010
+VERSION:2.0
+...
+END:VCALENDAR
+
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_--
+