[ESQL] String escaping fix (#196643)

## Summary

To PR fix the problem with string escaping, we need to ensure that
backslashes are properly escaped in addition to double quotes.

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

packages/kbn-esql-utils/src/utils/append_to_query.ts

@@ -40,7 +40,8 @@ export function appendWhereClauseToESQLQuery(
     default:
       operator = '==';
   }
-  let filterValue = typeof value === 'string' ? `"${value.replace(/\"/g, '\\"')}"` : value;
+  let filterValue =
+    typeof value === 'string' ? `"${value.replace(/\\/g, '\\\\').replace(/\"/g, '\\"')}"` : value;
   // Adding the backticks here are they are needed for special char fields
   let fieldName = `\`${field}\``;