[Obs AI Assistant] Enable syntax highlighting for ES|QL (#212669)

Closes https://github.com/elastic/kibana/issues/212384

## Summary

With https://github.com/elastic/eui/pull/8317, `EuiCodeBlock` supports
syntax highlighting for ES|QL.
This PR enables it for Obs AI Assistant

### Screenshots

Before:
![Screenshot 2025-02-27 at 11 00
39 AM](https://github.com/user-attachments/assets/ec9c6da9-c945-4a6b-b3aa-ece17533e877)

After:

![image](https://github.com/user-attachments/assets/7ed5232f-9b4c-45a3-a55f-22c2b6f56a98)

### 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-pack/platform/plugins/shared/observability_ai_assistant/public/components/message_panel/esql_code_block.tsx

@@ -61,10 +61,12 @@ export function CodeBlock({ children }: { children: React.ReactNode }) {
 
 export function EsqlCodeBlock({
   value,
+  lang,
   actionsDisabled,
   onActionClick,
 }: {
   value: string;
+  lang: string;
   actionsDisabled: boolean;
   onActionClick: ChatActionClickHandler;
 }) {
@@ -72,7 +74,7 @@ export function EsqlCodeBlock({
     <CodeBlockWrapper>
       <EuiFlexGroup direction="column" gutterSize="xs">
         <EuiFlexItem grow={false}>
-          <EuiCodeBlock isCopyable fontSize="m">
+          <EuiCodeBlock language={lang} isCopyable fontSize="m">
             {value}
           </EuiCodeBlock>
         </EuiFlexItem>

x-pack/platform/plugins/shared/observability_ai_assistant/public/components/message_panel/message_text.tsx

@@ -147,6 +147,7 @@ export function MessageText({ loading, content, onActionClick }: Props) {
           <>
             <EsqlCodeBlock
               value={props.value}
+              lang={props.lang}
               actionsDisabled={loading}
               onActionClick={onActionClickRef.current}
             />