`getGroupedRowModel` doesn't calculate `leafRows` correctly at depth 0

[stevenwjy]

TanStack Table version

8.21.3

Framework/Library version

React v19.0

Describe the bug and the steps to reproduce it

See the implementation here: https://github.com/TanStack/table/blob/9c62cf2024b86c419f7015f963248739bab1b0e7/packages/table-core/src/utils/getGroupedRowModel.ts#L75

              // Flatten the leaf rows of the rows in this group
              const leafRows = depth
                ? flattenBy(groupedRows, row => row.subRows)
                : groupedRows

The current implementation only calls the flattenBy function for depth > 0. Meanwhile, I believe the purpose of leafRows is to always create the flattened version and thus the flattenBy must always be called unless groupedRows no longer contains any subrows -- need to check groupedRows.some((row) => row.subRows.length > 0) instead of whether depth is falsy.

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

Screenshots or Videos (Optional)

No response

Do you intend to try to help solve this bug with your own PR?

Yes, I think I know how to fix it and will discuss it in the comments of this issue

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