test(channels): add assert for Signal default output formatter

crates/openfang-channels/src/bridge.rs

@@ -1933,6 +1933,10 @@ mod tests {
             default_output_format_for_channel("discord"),
             OutputFormat::Markdown
         );
+        assert_eq!(
+            default_output_format_for_channel("signal"),
+            OutputFormat::PlainText
+        )
     }
 
     #[tokio::test]