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 {

web/src/pages/agent/constant/index.tsx

@@ -135,6 +135,31 @@ export const initialMessageValues = {
   content: [''],
 };
 
+export const initialExcelProcessorValues = {
+  input_files: [],
+  operation: 'read',
+  sheet_selection: 'all',
+  merge_strategy: 'concat',
+  join_on: '',
+  transform_data: '',
+  output_format: 'xlsx',
+  output_filename: 'output',
+  outputs: {
+    data: {
+      type: 'object',
+      value: {},
+    },
+    summary: {
+      type: 'string',
+      value: '',
+    },
+    markdown: {
+      type: 'string',
+      value: '',
+    },
+  },
+};
+
 export const initialDuckValues = {
   top_n: 10,
   channel: Channel.Text,
@@ -832,6 +857,7 @@ export enum ExportFileType {
   HTML = 'html',
   Markdown = 'md',
   DOCX = 'docx',
+  Excel = 'xlsx',
 }
 
 export enum TypesWithArray {