fix: row grouping algorithm

[citizensas]

• Improved Row Index Mapping:
  Enhanced mapRowIndexToPath for edge cases and overflow.
• Enhanced useRowGrouping Hook:
  Refined mapper function for type safety.

[Copilot]

Pull Request Overview

This PR fixes errors in the row grouping algorithm to better handle edge cases and improve type safety.

• Refactors mapRowIndexToPath to fix an off‐by-one error in row mapping logic.
• Updates the RowGroupingMapperResult type usage and associated tests to ensure accurate row index mapping.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/core/test/row-grouping-api.test.ts	Adds comprehensive tests to verify row grouping behavior.
packages/core/src/data-editor/row-grouping.ts	Adjusts index calculations in the row-mapping algorithm.
packages/core/src/data-editor/row-grouping-api.ts	Refines type definitions for the row grouping mapper function.

Comments suppressed due to low confidence (2)

packages/core/src/data-editor/row-grouping.ts:203

• Consider adding a brief inline comment explaining the '-1' adjustment in the spread 'path: [...group.path, toGo - 1]'. This would improve clarity regarding how the offset is calculated.

if (toGo <= group.rows)

packages/core/src/data-editor/row-grouping-api.ts:12

• Avoid using 'as any' here by refining the type definitions of the mapper result; this would improve type safety and overall code maintainability.

return { ...r, originalIndex: [itemOrRow[0], r.originalIndex], } as any; // FIXME

[lukasmasuch]

LGTM 👍