[ML] AIOps: Field stats fix (elastic#204403)

## Summary

Fixes a regression introduced in
elastic#203579.

`EuiText` accepts EUI colors like `primary` as well as hex colors. The
PR above introduced `getPercentageColor()` which assumed all colors
passed in are hex colors. In case it was an EUI color token the code
threw an error and the fields stats popover wouldn't fully render. This
fix adds an additional check to apply `makeHighContrastColor` only if
the color provided is a hex color.

Before:

![CleanShot 2024-12-16 at 14 50
15@2x](https://github.com/user-attachments/assets/53a5df17-da4a-4188-a80c-5dc52cae9f39)

After:

![CleanShot 2024-12-16 at 14 49
20@2x](https://github.com/user-attachments/assets/75fc093a-6cf6-4a21-bc10-05dd7d253c70)

### Checklist

- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [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)

packages/kbn-unified-field-list/src/components/field_stats/field_top_values_bucket.tsx

@@ -211,8 +211,14 @@ const FieldTopValuesBucket: React.FC<FieldTopValuesBucketProps> = ({
   );
 };
 
+// copied from x-pack/plugins/banners/server/utils.ts
+const hexColorRegexp = /^#([0-9a-f]{6}|[0-9a-f]{3})$/i;
+const isHexColor = (color: string) => {
+  return hexColorRegexp.test(color);
+};
+
 const getPercentageColor = (euiTheme: EuiThemeComputed, color: string) =>
-  euiTheme.themeName?.toLowerCase().includes('borealis')
+  euiTheme.themeName?.toLowerCase().includes('borealis') && isHexColor(color)
     ? makeHighContrastColor(color)(euiTheme)
     : color; // FIXME: remove in 9.x when Borealis becomes the default theme