matrix: add #[must_use] on room_label per review

hive-matrix-mcp/src/handlers.rs

@@ -137,6 +137,7 @@ fn extract_body_sync(event: &matrix_sdk::ruma::events::AnySyncTimelineEvent) ->
 /// Human-facing label for a room: its canonical alias when set,
 /// otherwise the raw room id. Used wherever a room is named in
 /// agent-facing text (wake bodies, loose-ends, the unread guard).
+#[must_use]
 pub fn room_label(room: &matrix_sdk::Room) -> String {
     room.canonical_alias()
         .map_or_else(|| room.room_id().to_string(), |a| a.to_string())