Fix delete functionality for vendor notes dynamic rows

- Changed row ID prefix from 'row_' to '_' for valid CSS selectors
- Added _id property assignment in row data for proper rendering
- Resolves querySelectorAll SyntaxError when deleting rows

The issue was that numeric row IDs (0, 1, 2) created invalid CSS
selectors like 'tr#0 button.action-delete'. CSS IDs cannot start
with numbers. By prefixing with underscore, we get valid selectors
like 'tr#_0 button.action-delete'.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Block/Adminhtml/Form/Field/VendorMapping.php

@@ -50,10 +50,13 @@ class VendorMapping extends AbstractFieldArray
         $result = [];
         /** @var DataObject $row */
         foreach (parent::getArrayRows() as $key => $row) {
-            // Ensure row IDs are prefixed with a non-numeric character
+            // Ensure row IDs are prefixed with underscore to make valid CSS selectors
+            // CSS IDs cannot start with a number
             if (is_numeric($key)) {
-                $key = 'row_' . $key;
+                $key = '_' . $key;
             }
+            // Also set the _id property in the row data for proper rendering
+            $row->setData('_id', $key);
             $result[$key] = $row;
         }
         return $result;