[8.x] [ESQL] String escaping fix (#196643) (#196676)

# Backport

This will backport the following commits from `main` to `8.x`:
- [[ESQL] String escaping fix
(#196643)](#196643)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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String escaping fix (#196643)\n\n## Summary\r\n\r\nTo PR fix the problem
with string escaping, we need to ensure that\r\nbackslashes are properly
escaped in addition to double quotes.\r\n\r\nCo-authored-by: Copilot
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String escaping
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String escaping fix (#196643)\n\n## Summary\r\n\r\nTo PR fix the problem
with string escaping, we need to ensure that\r\nbackslashes are properly
escaped in addition to double quotes.\r\n\r\nCo-authored-by: Copilot
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String escaping fix (#196643)\n\n## Summary\r\n\r\nTo PR fix the problem
with string escaping, we need to ensure that\r\nbackslashes are properly
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Co-authored-by: Elena Shostak <[email protected]>

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}\``;