[9.2] [ObsUx][Hosts]Add missing "There are hosts available in another schema" message (#241275) (#241323)

# Backport

This will backport the following commits from `main` to `9.2`:
- [[ObsUx][Hosts]Add missing "There are hosts available in another
schema" message
(#241275)](#241275)

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

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

<!--BACKPORT
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missing \"There are hosts available in another schema\" message
(#241275)\n\nCloses
https://github.com/elastic/kibana/issues/239691\n\n## Summary\n\nWhen
someone runs a query in the Hosts UI, if there are hosts available\nin
another schema, there should be wording to tell them to switch
the\nschema.\n\nBEFORE\n\n<img width=\"1218\" height=\"571\"
alt=\"Screenshot 2025-10-30 at 12 13
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missing \"There are hosts available in another schema\"
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missing \"There are hosts available in another schema\" message
(#241275)\n\nCloses
https://github.com/elastic/kibana/issues/239691\n\n## Summary\n\nWhen
someone runs a query in the Hosts UI, if there are hosts available\nin
another schema, there should be wording to tell them to switch
the\nschema.\n\nBEFORE\n\n<img width=\"1218\" height=\"571\"
alt=\"Screenshot 2025-10-30 at 12 13
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missing \"There are hosts available in another schema\" message
(#241275)\n\nCloses
https://github.com/elastic/kibana/issues/239691\n\n## Summary\n\nWhen
someone runs a query in the Hosts UI, if there are hosts available\nin
another schema, there should be wording to tell them to switch
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alt=\"Screenshot 2025-10-30 at 12 13
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BACKPORT-->

Co-authored-by: Miriam <[email protected]>

Author: kibanamachine

x-pack/solutions/observability/plugins/infra/public/components/schema_selector.tsx

@@ -238,7 +238,7 @@ export const SchemaSelector = ({
           })}
           css={{ minWidth: isHostsView ? '400px' : '300px' }}
           helpText={
-            options.length > 1 &&
+            (options.length > 1 || (options.length === 1 && isInvalid)) &&
             i18n.translate('xpack.infra.schemaSelector.select.helpText', {
               defaultMessage: 'There are hosts available in another schema',
             })