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

Terms & Code of Conduct

• I agree to follow this project's Code of Conduct
• I understand that if my bug cannot be reliable reproduced in a debuggable environment, it will probably not be fixed and this issue may even be closed.

Start with the linked CodeSandbox and reproduce the nested-header case, then trace how leaf headers are calculated in the table code. Confirm whether headers with subheaders are included intentionally; done means either correcting the calculation and covering the behavior with a regression test, or documenting the intended definition of a leaf header.