Enhance multiselect option handling and states

[evg-smirnov]

Summary

Improve multiselect group and "Select All" state handling.

Changes

• Added support for the indeterminate state for option group checkboxes.
• Added support for the indeterminate state for the "Select All" checkbox when only part of the available options is selected.
• Updated group state calculation logic to track the number of selected child options instead of relying on a simple boolean flag.
• Reset the indeterminate state when toggling option groups.
• Updated option group template initialization to include the default collapsed state.

Benefits

• Provides more accurate visual feedback for partially selected groups.
• Keeps group and "Select All" controls synchronized with the actual selection state.
• Improves UX for large multiselects containing grouped options.

Testing

• Verified group checkbox behavior when:

  • no child options are selected;
  • some child options are selected;
  • all child options are selected.

• Verified "Select All" behavior for empty, partial, and full selections.

• Verified that clicking a group checkbox clears the indeterminate state and applies the expected selection state.