test: add unit tests for RhythmBlocks.js (#4965)

- Add module.exports to RhythmBlocks.js with proper check
- Create tests for all 24 registered rhythm blocks
- Verify NewSwing2Block default behaviors on error
- Ensure 100% test coverage for flow logic

Closes #4970

Author: vyagh

js/blocks/RhythmBlocks.js

@@ -1251,3 +1251,7 @@ function setupRhythmBlocks(activity) {
     new DefineFrequencyBlock().setup(activity);
     new OctaveSpaceBlock().setup(activity);
 }
+
+if (typeof module !== "undefined" && module.exports) {
+    module.exports = { setupRhythmBlocks };
+}