gh-35904: Make SubgraphSearch robust to vertex labels
Part of #35902.
### :books: Description
We ensure that the method operates properly even when vertices and edges
are of incomparable types.
On the way, we also avoid a call to `adjacency_matrix`.
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/35904
Reported by: David Coudert
Reviewer(s): Matthias Köppe
