[Security Assistant] Fix markdown syntax highlighting (elastic#212333)

## Summary
Implements elastic/eui#8317

Before:
<img width="1007" alt="image"
src="https://github.com/user-attachments/assets/649a075a-7338-434b-9e2e-4c4fd0d6ca6f"
/>

After:
<img width="1484" alt="image"
src="https://github.com/user-attachments/assets/cdb27885-e009-49aa-9b1a-381e12c33727"
/>

(cherry picked from commit f8ba372)

x-pack/solutions/security/plugins/security_solution/public/assistant/get_comments/custom_codeblock/custom_code_block.tsx

@@ -8,7 +8,7 @@ import { EuiCodeBlock, EuiFlexGroup, EuiFlexItem, EuiPanel, useEuiTheme } from '
 import { css } from '@emotion/css';
 import React from 'react';
 
-export const CustomCodeBlock = ({ value }: { value: string }) => {
+export const CustomCodeBlock = ({ value, lang }: { value: string; lang: string }) => {
   const theme = useEuiTheme();
 
   return (
@@ -31,7 +31,7 @@ export const CustomCodeBlock = ({ value }: { value: string }) => {
     >
       <EuiFlexGroup direction="column" gutterSize="xs">
         <EuiFlexItem grow={false}>
-          <EuiCodeBlock isCopyable fontSize="m">
+          <EuiCodeBlock isCopyable fontSize="m" language={lang}>
             {value}
           </EuiCodeBlock>
         </EuiFlexItem>

x-pack/solutions/security/plugins/security_solution/public/assistant/get_comments/stream/message_text.tsx

@@ -133,7 +133,7 @@ const getPluginDependencies = ({
     customCodeBlock: (props) => {
       return (
         <>
-          <CustomCodeBlock value={props.value} />
+          <CustomCodeBlock value={props.value} lang={props.lang} />
           <EuiSpacer size="m" />
         </>
       );