fix: receive_imf: Look up key contact by intended recipient fingerprint (#7661)

For now, do this only for `OneOneChat` and `MailingListOrBroadcast`, this is enough to correctly
support messages from modern Delta Chat versions sending Intended Recipient Fingerprint subpackets
and single-recipient messages from modern versions of other MUAs.

src/test_utils.rs

@@ -117,6 +117,8 @@ impl TestContextManager {
             .await
     }
 
+    /// Returns new elena's "device".
+    /// Elena doesn't send Intended Recipient Fingerprint subpackets to simulate old Delta Chat.
     pub async fn elena(&mut self) -> TestContext {
         TestContext::builder()
             .configure_elena()