[Discover Tabs] Visually connect the active tab with the top nav (#214440)

- Closes https://github.com/elastic/kibana/issues/210864

## Summary

This PR changes tabs styles and visually connects the selected tab with
the Kibana header.

Classic view:

<img width="1439" alt="Screenshot 2025-03-17 at 13 26 16"
src="https://github.com/user-attachments/assets/31dc0311-7bc1-4bc8-9b83-48f40227705f"
/>
<img width="1435" alt="Screenshot 2025-03-17 at 13 26 52"
src="https://github.com/user-attachments/assets/301963fb-3207-49ae-ab70-177834f3a73f"
/>


Project view:

<img width="1438" alt="Screenshot 2025-03-17 at 13 25 34"
src="https://github.com/user-attachments/assets/df1d1bff-82f8-4eed-9cf3-b3e557f5658c"
/>
<img width="1437" alt="Screenshot 2025-03-17 at 13 24 58"
src="https://github.com/user-attachments/assets/dfe27fc7-1cfe-4695-b1fd-2e306adc8787"
/>

### Testing

Two options are possible:

1. start Storybook with `yarn storybook unified_tabs` and navigate to
`http://localhost:9001`.
2. start Kibana with `yarn start --run-examples`. Then navigate to the
Unified Tabs example plugin
`http://localhost:5601/app/unifiedTabsExamples`.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

examples/unified_tabs_examples/public/example_app.tsx

@@ -99,6 +99,7 @@ export const UnifiedTabsExampleApp: React.FC<UnifiedTabsExampleAppProps> = ({
             <UnifiedTabs
               initialItems={initialItems}
               maxItemsCount={25}
+              services={services}
               onChanged={() => {}}
               createItem={getNewTabDefaultProps}
               renderContent={({ label }) => {