fix: Drop messages encrypted with the wrong symmetric secret (#7963)

The tests were originally generated with AI and then reworked.

Follow-up to https://github.com/chatmail/core/pull/7754 (c724e29)

This prevents the following attack:

/// Eve is subscribed to a channel and wants to know whether Alice is also subscribed to it.
/// To achieve this, Eve sends a message to Alice
/// encrypted with the symmetric secret of this broadcast channel.
///
/// If Alice sends an answer (or read receipt),
/// then Eve knows that Alice is in the broadcast channel.
///
/// A similar attack would be possible with auth tokens
/// that are also used to symmetrically encrypt messages.
///
/// To prevent this, a message that was unexpectedly
/// encrypted with a symmetric secret must be dropped.

src/mimeparser/mimeparser_tests.rs

@@ -2171,9 +2171,6 @@ async fn test_load_shared_secrets_with_legacy_state() -> Result<()> {
         ()
     ).await?;
 
-    // This call must not fail:
-    load_shared_secrets(alice).await.unwrap();
-
     let qr: QrInvite = alice
         .sql
         .query_get_value("SELECT invite FROM bobstate", ())