add unit test coverage for AggregationOperation

[aglinxinyuan]

Task Summary

AggregateOpSpec already covers getAggregationAttribute for SUM, COUNT, and CONCAT. Extend it with the remaining gaps so the full mapping is pinned in one place:

• AVERAGE always produces DOUBLE
• MIN preserves the input type
• MAX preserves the input type
• A null aggFunction raises RuntimeException

Priority

P3 – Low

Task Type

• Testing / QA