[ML] Add aria-label to Anomaly score button for screen reader accessibility (elastic#266469)

The Anomaly score button in the Anomaly Explorer only announces its
selection value ("Multiple") to screen readers, missing the "Anomaly
score" title context. This violates WCAG SC 4.1.2 (Name, Role, Value).

Fixes elastic#264988

### Changes

- Added optional `aria-label` prop to `MultiSuperSelect` component,
forwarded to the inner `EuiButtonEmpty`
- Passed the "Anomaly score" label as `aria-label` in `SelectSeverityUI`
so assistive technology announces the full context

Screen readers will now announce: *"Anomaly score, Multiple, collapsed,
button"* instead of just *"Multiple, collapsed, button"*.

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `ci-stats.kibana.dev`
> - Triggering command:
`/home/REDACTED/.nvm/versions/node/v24.14.1/bin/node
/home/REDACTED/.nvm/versions/node/v24.14.1/bin/node
scripts/yarn_install_scripts.js run x86-64.so.2 0.8.2 if (a[i] &lt;`
(dns block)
> - Triggering command:
`/home/REDACTED/.nvm/versions/node/v24.14.1/bin/node
/home/REDACTED/.nvm/versions/node/v24.14.1/bin/node scripts/kbn
bootstrap` (dns block)
> - Triggering command:
`/home/REDACTED/.nvm/versions/node/v24.14.1/bin/node node
scripts/check_changes.ts dd ldd s/li��` (dns block)
> - `clients3.google.com`
> - Triggering command:
`/home/REDACTED/work/kibana/kibana/node_modules/@moonrepo/core-linux-x64-gnu/moon
/home/REDACTED/work/kibana/kibana/node_modules/@moonrepo/core-linux-x64-gnu/moon
run :build-webpack ldd 0.8.2` (dns block)
> - `detectportal.firefox.com`
> - Triggering command:
`/home/REDACTED/work/kibana/kibana/node_modules/@moonrepo/core-linux-x64-gnu/moon
/home/REDACTED/work/kibana/kibana/node_modules/@moonrepo/core-linux-x64-gnu/moon
run :build-webpack ldd 0.8.2` (dns block)
> - `google.com`
> - Triggering command:
`/home/REDACTED/work/kibana/kibana/node_modules/@moonrepo/core-linux-x64-gnu/moon
/home/REDACTED/work/kibana/kibana/node_modules/@moonrepo/core-linux-x64-gnu/moon
run :build-webpack ldd 0.8.2` (dns block)
> - `googlechromelabs.github.io`
> - Triggering command:
`/home/REDACTED/.nvm/versions/node/v24.14.1/bin/node
/home/REDACTED/.nvm/versions/node/v24.14.1/bin/node install.js
_TOKEN&elastic#34;; }; f ldd b/li��
nibrowser-gtk/sys/lib/libbrotlienc.so.1.0.7` (dns block)
> - `iojs.org`
> - Triggering command: `/usr/bin/curl curl -q --fail --compressed -L -s
REDACTED -o -` (dns block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/elastic/kibana/settings/copilot/coding_agent)
(admins only)
>
> </details>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: L1nBra <178042654+L1nBra@users.noreply.github.com>
Co-authored-by: Bhavya RM <bhavya@elastic.co>
Co-authored-by: L1nBra <lina.braziulyte@gmail.com>
Co-authored-by: Alexey Antonov <alexwizp@gmail.com>

Author: 4 people

x-pack/platform/plugins/shared/ml/public/application/components/controls/select_severity/select_severity.tsx

@@ -237,12 +237,15 @@ export const SelectSeverityUI: FC<
     [onChange, allSeverityOptions]
   );
 
+  const anomalyScoreLabel = i18n.translate('xpack.ml.explorer.severityThresholdLabel', {
+    defaultMessage: 'Anomaly score',
+  });
+
   return (
     <div data-test-subj={'mlAnomalySeverityThresholdControls'} className={classNames}>
       <MultiSuperSelect
-        prepend={i18n.translate('xpack.ml.explorer.severityThresholdLabel', {
-          defaultMessage: 'Anomaly score',
-        })}
+        prepend={anomalyScoreLabel}
+        aria-label={anomalyScoreLabel}
         inputDisplay={inputDisplay}
         options={multiSelectOptions}
         onOptionsChange={handleOptionsChange}

x-pack/platform/plugins/shared/ml/public/application/components/multi_super_select/multi_super_select.tsx

@@ -15,13 +15,15 @@ interface MultiSuperSelect<T> {
   inputDisplay: React.JSX.Element | string;
   options: Array<EuiSelectableOption<T>>;
   onOptionsChange?: (options: Array<EuiSelectableOption<T>>) => void;
+  'aria-label'?: string;
 }
 
 export const MultiSuperSelect = <T = string,>({
   prepend,
   inputDisplay,
   options,
   onOptionsChange,
+  'aria-label': ariaLabel,
 }: MultiSuperSelect<T>) => {
   const [isPopoverOpen, setIsPopoverOpen] = useState(false);
 
@@ -33,7 +35,7 @@ export const MultiSuperSelect = <T = string,>({
 
   const popoverButton = (
     <EuiFormControlLayout isDropdown prepend={prepend} compressed fullWidth>
-      <EuiButtonEmpty onClick={onPopoverButtonClick} css={styles.control}>
+      <EuiButtonEmpty onClick={onPopoverButtonClick} css={styles.control} aria-label={ariaLabel}>
         {inputDisplay}
       </EuiButtonEmpty>
     </EuiFormControlLayout>