[Callout] Fix dismiss icon and sub-pixel border

[ravisuhag]

Summary

Improvements to the Callout component to fix patterns and clean up CSS.

Improvements

5. Inline SVG for dismiss icon

The dismiss button renders an inline SVG path. Should use a shared icon component for consistency and bundle size.

7. Sub-pixel border value

Uses border: 0.5px solid which may render inconsistently across browsers/displays.

Moved to Cross-Cutting Issues

The following items have been moved to cross-cutting issues that address them across all components:

• Replace hardcoded CSS values with design tokens (Replace hardcoded CSS values with design tokens #672)
• Clean up deprecated code (Clean up deprecated code, unused CSS, and legacy patterns #682)
• Consistent className merging (Consistent className merging using cx()/cn() #683)