Leaf rows are not calculated correctly

TanStack Table version

v8.19.3

Framework/Library version

v.18.0.4

Describe the bug and the steps to reproduce it

Bug exists also in the newest version. Leaf headers include headers which contain subheaders - it seems inconsistent with a definition of a leaf node. Quoting https://en.wikipedia.org/wiki/Tree_(graph_theory):

[...] an external vertex (or outer vertex, terminal vertex or leaf) is a vertex of degree 1.

Which means one edge connected. With that logic, only headers with no subheaders should be treated as leaf headers - the only exception is the root header with one subheader. If this is a design choice, not a feature, I think an understanding of a leaf should be included in the docs, as it is at least confusing.

I'm not opening a PR because I'm not sure if I'm not missing something, but if it turns out to be an issue I'm happy to fix it.

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

https://codesandbox.io/p/sandbox/h684m4

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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