fix(button-group): fix button group colors in selected state

[didimmova]

Closes #17221

test with this theming pr - IgniteUI/igniteui-theming#571

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog
• Skills/Agents

Component(s) / Area(s) Affected:

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Test Configuration:

• Angular version:
• Browser(s):
• OS:

Screenshots / Recordings

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified

[Copilot]

Pull request overview

Fixes incorrect border coloring for selected igx-button-group items in the focused/hovered focused states, aligning the selected-state visuals with the intended theme tokens (per the linked theming PR and issue).

Changes:

• Use item-selected-border-color (instead of item-selected-hover-border-color) for the selected + focused state border.
• Use item-selected-hover-border-color for the selected + focused + hovered state border (instead of item-selected-border-color).
• Remove now-redundant Indigo-only border-color overrides that matched the new generalized behavior.