gh-41038: some care about algebra_generators
    
add some type annotations

add a check for finite length in the `gens` method for the sake of
python 3.14 change of behaviour in `tuple`

related to  #41028 for Jordan algebras

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.
    
URL: #41038
Reported by: Frédéric Chapoton
Reviewer(s): Chenxin Zhong, Frédéric Chapoton, Travis Scrimshaw

Author: Release Manager