Add tests for Greatest and Least functions

This commit adds test cases for both Greatest and Least functions to verify:
- All supported data types (boolean, double, integer, long, keyword, text, version, ip, datetime, date_nanos)
- NULL value handling
- String comparisons
- Numeric comparisons
- Boolean operations
- Date/time comparisons

The tests ensure that:
- Both functions handle NULL values correctly
- Type validation works as expected
- Evaluator selection is correct for each data type
- Results are computed correctly for all supported types

Relates to #114036

Author: shinsj4653

docs/changelog/128429.yaml

@@ -0,0 +1,6 @@
+pr: 127949
+summary: Refactor Greatest and Least functions to use evaluator map
+area: ES|QL
+type: bug
+issues:
+  - 114036