Preserve constant values in union operations

[gokselk]

Is your feature request related to a problem or challenge?

When performing union operations in DataFusion, constant values are not preserved across partitions even when they are identical on both sides. This limits optimization opportunities.

Describe the solution you'd like

Add constant value tracking to ConstExpr and preserve constant values across union operations when both sides have matching constants. This will enable better query optimization.

Describe alternatives you've considered

Continue with current behavior where constants are not preserved across unions, but this misses optimization opportunities.

Additional context

This enhancement will help with query optimization by maintaining constant value information across union operations when the values match on both sides.