[Lens] Increase z-index for formula widget (elastic#244850)

## Summary

Fix elastic#244283

Makes the Lens formula auto suggestions visible during inline Lens
editing in dashboards.

<img width="1724" height="1279" alt="Screenshot 2025-12-02 at 08 54 48"
src="https://github.com/user-attachments/assets/a3b84e47-2188-45cf-930d-e7cbb923db3d"
/>

### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [x] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

(cherry picked from commit 1cdb12c)

Author: mariairiartef

x-pack/platform/plugins/shared/lens/public/datasources/form_based/operations/definitions/formula/editor/formula_editor.tsx

@@ -12,6 +12,7 @@ import {
   LanguageDocumentationPopoverContent,
 } from '@kbn/language-documentation';
 import { css } from '@emotion/react';
+import { css as cssClassName } from '@emotion/css';
 import {
   EuiButtonIcon,
   EuiButtonEmpty,
@@ -158,7 +159,11 @@ export function FormulaEditor({
     const node1 = (overflowDiv1.current = document.createElement('div'));
     node1.setAttribute('data-test-subj', 'lnsFormulaWidget');
     // Monaco CSS is targeted on the monaco-editor class
-    node1.classList.add('lnsFormulaOverflow', 'monaco-editor');
+    node1.classList.add(
+      'lnsFormulaOverflow',
+      'monaco-editor',
+      cssClassName(sharedEditorStyles.overflowWidget(euiThemeContext))
+    );
     document.body.appendChild(node1);
   }
 
@@ -983,18 +988,17 @@ const sharedEditorStyles = {
         }
       }
 
-      .lnsFormulaOverflow {
-        // Needs to be higher than the modal and all flyouts
-        z-index: ${euiTheme.levels.toast} + 1;
-      }
-
       .lnsFormula__editorContent {
         background-color: ${euiTheme.colors.backgroundBasePlain};
         min-height: 0;
         position: relative;
       }
     `;
   },
+  overflowWidget: ({ euiTheme }: UseEuiTheme) => `
+    // Needs to be higher than the modal and all flyouts
+    z-index: ${Number(euiTheme.levels.toast) + 1};
+  `,
   formulaDocs: ({ euiTheme }: UseEuiTheme) => css`
     display: flex;
     flex-direction: column;