[Search][A11y] Search application disable add selected button when error in selected indices (elastic#241031)

## Summary

Fix A11y issue. When there is an error in the search application -> add
new indices-> select indices combo box, add selected button should be
disabled.

**Preconditions**
Stateful Search Applications -> Content page is opened.
Search application is added.
I used Mac voice over or Screen Reader (NVDA). 

**To reproduce:** 

1. Using only keyboard Navigate to Add new indices button by pressing
Tab key.
2. Press Enter.
3. Navigate to Select searchable indices field.
4. Enter any text (f.e.: test).
5. Navigate to Add selected button.
6. Press Enter.
7. Observe screen reader.
8. Press Tab key.
9. Observe the page.

### Screenshot
<img width="2052" height="1250" alt="Screenshot 2025-10-28 at 3 51
07 PM"
src="https://github.com/user-attachments/assets/c07c5c75-f3ca-47b9-ae2f-6ba2e972d103"
/>


### Checklist

- [x] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)

Author: saarikabhasi

x-pack/solutions/search/plugins/enterprise_search/public/applications/applications/components/search_application/add_indices_flyout.tsx

@@ -5,7 +5,7 @@
  * 2.0.
  */
 
-import React, { useCallback, useMemo } from 'react';
+import React, { useCallback, useMemo, useState } from 'react';
 
 import { useActions, useValues } from 'kea';
 
@@ -63,6 +63,8 @@ export const AddIndicesFlyout: React.FC<AddIndicesFlyoutProps> = ({ onClose }) =
     [setSelectedIndices]
   );
 
+  const [isIndicesSelectComboBoxDisabled, setIndicesSelectComboBoxDisabled] =
+    useState<boolean>(false);
   return (
     <EuiFlyout onClose={onClose} aria-labelledby={modalTitleId}>
       <EuiFlyoutHeader hasBorder>
@@ -102,6 +104,7 @@ export const AddIndicesFlyout: React.FC<AddIndicesFlyoutProps> = ({ onClose }) =
             'xpack.enterpriseSearch.searchApplications.searchApplication.indices.addIndicesFlyout.selectableLabel',
             { defaultMessage: 'Select searchable indices' }
           )}
+          setIndicesSelectComboBoxDisabled={setIndicesSelectComboBoxDisabled}
         />
       </EuiFlyoutBody>
       <EuiFlyoutFooter>
@@ -113,6 +116,7 @@ export const AddIndicesFlyout: React.FC<AddIndicesFlyoutProps> = ({ onClose }) =
               data-telemetry-id="entSearchApplications-indices-addNewIndices-submit"
               iconType="plusInCircle"
               onClick={submitSelectedIndices}
+              disabled={isIndicesSelectComboBoxDisabled}
             >
               {i18n.translate(
                 'xpack.enterpriseSearch.searchApplications.searchApplication.indices.addIndicesFlyout.submitButton',

x-pack/solutions/search/plugins/enterprise_search/public/applications/applications/components/search_applications/components/indices_select_combobox.tsx

@@ -37,13 +37,22 @@ export type IndicesSelectComboBoxProps = Omit<
   'data-telemetry-id'?: string;
   ignoredOptions?: string[];
   label?: string;
+  setIndicesSelectComboBoxDisabled?: (isInvalid: boolean) => void;
 };
 
-export const IndicesSelectComboBox = ({ ignoredOptions, ...props }: IndicesSelectComboBoxProps) => {
+export const IndicesSelectComboBox = ({
+  setIndicesSelectComboBoxDisabled,
+  ignoredOptions,
+  ...props
+}: IndicesSelectComboBoxProps) => {
   const [searchQuery, setSearchQuery] = useState<string | undefined>(undefined);
   const { makeRequest } = useActions(FetchIndicesForSearchApplicationsAPILogic);
   const { status, data } = useValues(FetchIndicesForSearchApplicationsAPILogic);
   const isInvalid = Boolean(searchQuery && !props.selectedOptions?.length);
+  useEffect(() => {
+    if (!setIndicesSelectComboBoxDisabled) return;
+    setIndicesSelectComboBoxDisabled(isInvalid);
+  }, [isInvalid]);
 
   useEffect(() => {
     makeRequest({ searchQuery });