Topologically sorted order for `list_available_variables`

**Is your feature request related to a problem? Please describe.**
People want this -- it's a nice guarantee to have.

**Describe the solution you'd like**
`list_available_variables` has topological sort documented nad observed. No ordering specified now so we can make it more specific.

**Describe alternatives you've considered**
- add a topological sort utility function
- add this to `HamiltonGraph`

**Additional context**
See [slack](https://hamilton-opensource.slack.com/archives/C03A9N36R5L/p1741946861506459)