[CLEAN] Synthetic Benchmark PR #139664 - ES|QL: Add MV_UNION Function

[tomerqodo]

Benchmark PR elastic#139664

Type: Clean (correct implementation)

Original PR Title: ES|QL: Add MV_UNION Function
Original PR Description: related: elastic#139298
Description:

  Adds MV_UNION function to ES|QL.
  
  Returns all unique values from both input multi-valued fields (set union).
  
  Example:
  Given set A = [1, 2, 3] and set B = [2, 3, 4]
  MV_UNION(A, B) returns [1, 2, 3, 4]

**Original PR URL**: https://github.com/elastic/elasticsearch/pull/139664