feat:Implement single column aggregations, Implement group by statments, implement having statments

[Rich-T-kid]

single column aggregations are implemented and tested, Next step is to include group by statments (having is already implented due to how flexible out expression interface is)

[Copilot]

Pull request overview

This PR implements single-column aggregation operations (MIN, MAX, COUNT, SUM, AVG) without GROUP BY support, consolidating previous stub implementations into a unified singleAggr.go file with comprehensive test coverage.

Key Changes

• Implemented accumulator-based aggregation pattern with 5 aggregate functions (MIN, MAX, COUNT, SUM, AVG)
• Created AggrExec operator that acts as a pipeline breaker, consuming all input batches and returning a single result row
• Added comprehensive test suite covering all aggregate functions, type validation, and error cases

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
singleAggr.go	New unified aggregation implementation with accumulator pattern and AggrExec operator
singleAggr_test.go	Comprehensive test suite for all aggregate functions and edge cases
sum.go, sum_test.go	Removed - consolidated into singleAggr.go
basicAggr.go, basicAggr_test.go	Removed - consolidated into singleAggr.go
avgExec.go, avgExec_test.go	Removed - consolidated into singleAggr.go
expr.go	Removed unnecessary comment from cast error handling

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[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 11 comments.

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[Copilot]

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated 21 comments.

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[Rich-T-kid]

introduce this embedding to all operators to reduce code duplications. seems like anice use case for golang

    child  operators.Operator
    schema *arrow.Schema
    done   bool
}