feat: impl handle_child_pushdown_result for UnionExec

[haohuaijin]

Which issue does this PR close?

• Closes Pushdown filter though UnionExec when some child support pushdown and some child do not support #20144

Rationale for this change

see #20144

What changes are included in this PR?

This PR impl handle_child_pushdown_result for UnionExec, for any case, the filter will always pushdown to UnionExec

UnionExec needs specialized filter pushdown handling when children have heterogeneous pushdown support. Without this, when some children support pushdown and others don't, the default behavior would leave FilterExec above UnionExec, re-applying filters to outputs of all children—including those that already applied the filters via pushdown. This specialized implementation adds FilterExec only to children that don't support pushdown, avoiding redundant filtering and improving performance.

Example: Given Child1 (no pushdown support) and Child2 (has pushdown support)
Default behavior:          This implementation:
FilterExec                 UnionExec
  UnionExec                  FilterExec
    Child1                     Child1
    Child2(filter)           Child2(filter)

Are these changes tested?

yes, add two test cases

Are there any user-facing changes?

[haohuaijin]

Thanks for your reviews @adriangb , i add the slt test case for reproduce.