fix(horizon-chart): use explicit array join for React key

Replace double cast `as unknown as string` with explicit `row.key.join(',')`
to properly convert the string array to a valid React key.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rusackas

superset-frontend/plugins/legacy-plugin-chart-horizon/src/HorizonChart.tsx

@@ -118,7 +118,7 @@ class HorizonChart extends PureComponent<HorizonChartProps> {
         >
           {data.map(row => (
             <HorizonRow
-              key={row.key as unknown as string}
+              key={row.key.join(',')}
               width={width}
               height={seriesHeight}
               title={ensureIsArray(row.key).join(', ')}