Add support for DataFrame sum operation with tests

[zaleslaw]

No description provided.

[Copilot]

Pull Request Overview

This PR adds support for the DataFrame sum operation along with comprehensive tests covering various summation scenarios and aggregation handlers. Key changes include:

• Adding a new sum operation test in both generated tests and dedicated test data for verifying correct summation.
• Extending the aggregation framework with new Sum0 and Sum1 implementations.
• Changing visibility modifiers from internal to public in several core aggregator components.

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
plugins/kotlin-dataframe/tests-gen/org/jetbrains/kotlin/fir/dataframe/DataFrameBlackBoxCodegenTestGenerated.java	Adds a new test method for the sum operation
plugins/kotlin-dataframe/testData/box/sum.kt	Introduces test scenarios for sum over all, selective, and expression-based columns
plugins/kotlin-dataframe/src/org/jetbrains/kotlinx/dataframe/plugin/loadInterpreter.kt	Updates load interpreter to support new Sum aggregators
plugins/kotlin-dataframe/src/org/jetbrains/kotlinx/dataframe/plugin/impl/api/statistics.kt	Adds implementations for summation aggregators
core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/statistics.kt	Expands tests for groupBy operations to include new numerical columns
core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/sum.kt	Annotates and exposes the new sum APIs
Other core aggregator files	Adjusts visibility and typing to support public sum operation API

Comments suppressed due to low confidence (1)

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/statistics.kt:571

• In the groupBy maxBy test, the column names are checked using 'res4' while the aggregation result is later accessed from 'res5'. This inconsistency might lead to erroneous test behavior; ensure the same result object is used throughout the test.

res4.columnNames() shouldBe listOf("city", "name", "age", "weight", "height", "yearsToRetirement", "workExperienceYears", "dependentsCount", "annualIncome")