fix: Add messages that can't be verified as DownloadState::Available (#7059)

We haven't dropped verified groups yet, so we need to do smth with messages that can't be verified
yet which often occurs because of messages reordering, particularly in large groups. Apart from the
reported case #7059, i had other direct reports that sometimes messages can't be verified for
various reasons, but when the reason is already fixed, it should be possible to re-download failed
messages and see them.

Also remove the code replacing the message text with a verification error from
`apply_group_changes()` as `add_parts()` already does this.

src/receive_imf/receive_imf_tests.rs

@@ -5091,6 +5091,44 @@ async fn test_two_group_securejoins() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_unverified_member_msg() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let bob = &tcm.bob().await;
+    let fiona = &tcm.fiona().await;
+
+    let alice_chat_id =
+        chat::create_group_chat(alice, ProtectionStatus::Protected, "Group").await?;
+    let qr = get_securejoin_qr(alice, Some(alice_chat_id)).await?;
+
+    tcm.exec_securejoin_qr(bob, alice, &qr).await;
+    tcm.exec_securejoin_qr(fiona, alice, &qr).await;
+
+    let fiona_chat_id = fiona.get_last_msg().await.chat_id;
+    let fiona_sent_msg = fiona.send_text(fiona_chat_id, "Hi").await;
+
+    // The message can't be verified, but the user can re-download it.
+    let bob_msg = bob.recv_msg(&fiona_sent_msg).await;
+    assert_eq!(bob_msg.download_state, DownloadState::Available);
+    assert!(
+        bob_msg
+            .text
+            .contains("Re-download the message or see 'Info' for more details")
+    );
+
+    let alice_sent_msg = alice
+        .send_text(alice_chat_id, "Hi all, it's Alice introducing Fiona")
+        .await;
+    bob.recv_msg(&alice_sent_msg).await;
+
+    // Now Bob has Fiona's key and can verify the message.
+    let bob_msg = bob.recv_msg(&fiona_sent_msg).await;
+    assert_eq!(bob_msg.download_state, DownloadState::Done);
+    assert_eq!(bob_msg.text, "Hi");
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_sanitize_filename_in_received() -> Result<()> {
     let alice = &TestContext::new_alice().await;