✨ feat(message_list): create message summary struct

- introduce MessageSummary struct to encapsulate message id and subject
- implement methods to set and retrieve subject, with default if missing

src/message_list/message_summary.rs

@@ -0,0 +1,30 @@
+#[derive(Debug)]
+pub(crate) struct MessageSummary {
+    id: String,
+    subject: Option<String>,
+}
+
+impl MessageSummary {
+    pub(crate) fn new(id: &str) -> Self {
+        MessageSummary {
+            id: id.to_string(),
+            subject: None,
+        }
+    }
+
+    pub(crate) fn id(&self) -> &str {
+        &self.id
+    }
+
+    pub(crate) fn set_subject(&mut self, subject: &str) {
+        self.subject = Some(subject.to_string())
+    }
+
+    pub(crate) fn subject(&self) -> &str {
+        if let Some(s) = &self.subject {
+            s
+        } else {
+            "*** No Subject for Message ***"
+        }
+    }
+}