[8.x] [Obs AI Assistant] Fix page height of the AI Assistant app in solution views (elastic#215646) (elastic#215678)

# Backport

This will backport the following commits from `main` to `8.x`:
- [[Obs AI Assistant] Fix page height of the AI Assistant app in
solution views (elastic#215646)](elastic#215646)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Viduni
Wickramarachchi","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-03-24T12:16:46Z","message":"[Obs
AI Assistant] Fix page height of the AI Assistant app in solution views
(elastic#215646)\n\nCloses
https://github.com/elastic/kibana/issues/215637\n\n## Summary\n\nIn
solution views the AI Assistant page doesn't take up the full\nviewport
height because a hardcoded value is used.\nThis PR fixes this issue by
using `kbn/eui` variables instead.\n\nBefore:\n\n<img width=\"1619\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/5c94c325-1a15-49da-b162-beb15431a732\"\n/>\n\nAfter:\n\n<img
width=\"1621\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/88e0610e-cbfb-4266-a0fa-c19f031bbb93\"\n/>\n\n\n###
Checklist\n\n- [x] The PR description includes the appropriate Release
Notes section,\nand the correct `release_note:*` label is applied per
the\n[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)","sha":"505405df5c49e2166883a27baed948f2868b9e1a","branchLabelMapping":{"^v9.1.0$":"main","^v8.19.0$":"8.x","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:fix","Team:Obs
AI Assistant","backport:version","v9.1.0","v8.19.0"],"title":"[Obs AI
Assistant] Fix page height of the AI Assistant app in solution
views","number":215646,"url":"https://github.com/elastic/kibana/pull/215646","mergeCommit":{"message":"[Obs
AI Assistant] Fix page height of the AI Assistant app in solution views
(elastic#215646)\n\nCloses
https://github.com/elastic/kibana/issues/215637\n\n## Summary\n\nIn
solution views the AI Assistant page doesn't take up the full\nviewport
height because a hardcoded value is used.\nThis PR fixes this issue by
using `kbn/eui` variables instead.\n\nBefore:\n\n<img width=\"1619\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/5c94c325-1a15-49da-b162-beb15431a732\"\n/>\n\nAfter:\n\n<img
width=\"1621\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/88e0610e-cbfb-4266-a0fa-c19f031bbb93\"\n/>\n\n\n###
Checklist\n\n- [x] The PR description includes the appropriate Release
Notes section,\nand the correct `release_note:*` label is applied per
the\n[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)","sha":"505405df5c49e2166883a27baed948f2868b9e1a"}},"sourceBranch":"main","suggestedTargetBranches":["8.x"],"targetPullRequestStates":[{"branch":"main","label":"v9.1.0","branchLabelMappingKey":"^v9.1.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/215646","number":215646,"mergeCommit":{"message":"[Obs
AI Assistant] Fix page height of the AI Assistant app in solution views
(elastic#215646)\n\nCloses
https://github.com/elastic/kibana/issues/215637\n\n## Summary\n\nIn
solution views the AI Assistant page doesn't take up the full\nviewport
height because a hardcoded value is used.\nThis PR fixes this issue by
using `kbn/eui` variables instead.\n\nBefore:\n\n<img width=\"1619\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/5c94c325-1a15-49da-b162-beb15431a732\"\n/>\n\nAfter:\n\n<img
width=\"1621\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/88e0610e-cbfb-4266-a0fa-c19f031bbb93\"\n/>\n\n\n###
Checklist\n\n- [x] The PR description includes the appropriate Release
Notes section,\nand the correct `release_note:*` label is applied per
the\n[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)","sha":"505405df5c49e2166883a27baed948f2868b9e1a"}},{"branch":"8.x","label":"v8.19.0","branchLabelMappingKey":"^v8.19.0$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Co-authored-by: Viduni Wickramarachchi <[email protected]>

Author: kibanamachine

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 }) {