Feature/excel export fix (#11914)

### PR details 
feat: Add Excel export support and fix variable reference regex
Changes:
- Add Excel export output format option to Message component
- Apply nest_asyncio patch to handle nested event loops
- Fix async generator iteration in canvas_app.py debug endpoint
- Add underscore support in variable reference regex pattern


### What problem does this PR solve?



### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Shivam Johri <shivamjohri@Shivams-MacBook-Air.local>

web/src/constants/agent.tsx

@@ -116,6 +116,7 @@ export enum Operator {
   Loop = 'Loop',
   LoopStart = 'LoopItem',
   ExitLoop = 'ExitLoop',
+  ExcelProcessor = 'ExcelProcessor',
 }
 
 export enum ComparisonOperator {