gh-35896: deal with bad chosen representations of k-regular sequences
### :books: Description
It deals with the situation when mu[0]*right != right in k-regular
sequences.
Fixes #21343. This PR is created from the branch/code that has been on
the corresponding trac ticket + merging in a current SageMath version.
See also meta issue #21202.
### :memo: Checklist
- [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
- #21204: guessing k-regular sequences from the first few values
No formal dependencies, but there are trivial merge conflicts---in both
branches, methods are inserted at the same position in the code---
between:
- #21343: deal with mu[0]*right != right in k-regular sequences
- #35894: Regular sequences: implement convolution / ring structure
URL: https://github.com/sagemath/sage/pull/35896
Reported by: Daniel Krenn
Reviewer(s): cheuberg, Daniel Krenn
