[Obs AI Assistant] Fix page height of the AI Assistant app in solution views (#215646)

Closes #215637

## Summary

In solution views the AI Assistant page doesn't take up the full
viewport height because a hardcoded value is used.
This PR fixes this issue by using `kbn/eui` variables instead.

Before:

<img width="1619" alt="image"
src="https://github.com/user-attachments/assets/5c94c325-1a15-49da-b162-beb15431a732"
/>

After:

<img width="1621" alt="image"
src="https://github.com/user-attachments/assets/88e0610e-cbfb-4266-a0fa-c19f031bbb93"
/>


### 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)

Author: viduni94

x-pack/solutions/observability/plugins/observability_ai_assistant_app/public/components/page_template.tsx

@@ -14,7 +14,7 @@ const pageSectionContentClassName = css`
   flex-grow: 1;
   padding-top: 0;
   padding-bottom: 0;
-  max-block-size: calc(100vh - 96px);
+  max-block-size: calc(100vh - var(--kbnAppHeadersOffset, var(--euiFixedHeadersOffset, 0)));
 `;
 
 export function ObservabilityAIAssistantPageTemplate({ children }: { children: React.ReactNode }) {