Follow up: Fix SegmentButton and SelectButton loading spinner colors

[georgewrmarshall]

Overview

Follow-up from #1172. SegmentButton and SelectButton can pass an empty string into ButtonBase's loading spinner color path.

Problem

ButtonBase derives the loading spinner color from textClassName(pressed) when textClassName is provided:

color={
  textClassName
    ? (textClassName(pressed) as IconColor)
    : IconColor.IconDefault
}

The new components sometimes return '' from textClassName / iconClassName:

• SegmentButton: unselected states return ''
• SelectButton: tertiary variant returns ''

Because '' is still passed as an explicit color prop, it bypasses Spinner's IconColor.IconDefault default and leaves the loading icon without a valid design-system color class.

Acceptance Criteria

• Loading spinner color is valid for every SegmentButton variant/selected combination.
• Loading spinner color is valid for every SelectButton variant.
• Tests cover the loading spinner color for the affected unselected/tertiary states.

References

• PR: feat: Added SelectButton, SegmentButton, and SegmentGroup to DSRN #1172
• Current code:
  • packages/design-system-react-native/src/components/SegmentButton/SegmentButton.tsx
  • packages/design-system-react-native/src/components/SelectButton/SelectButton.tsx
  • packages/design-system-react-native/src/components/ButtonBase/ButtonBase.tsx