♻️ refactor(gmail_client): move message_summary to gmail_client

- move `message_summary.rs` from `message_list` to `gmail_client` for better organization
- derive `Clone` for `MessageSummary` struct

src/gmail_client/message_summary.rs

@@ -1,4 +1,4 @@
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub(crate) struct MessageSummary {
     id: String,
     subject: Option<String>,