main branch: bugfix/sparsejac-46

[c-randall]

Description

Fixes bug where the user-defined jacfn option was being ignored when linsolver='sparse'. The issue has already been resolved in the v1.1.x maintenance branch, and a patched release v1.1.2 is already on PyPI and conda-forge. This PR ensures the main branch also has the fix. Per the maintenance policy, no other older versions will receive this patch. Only the latest release and any releases done within the last year are eligible for bug fixes.

Fixes #46

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that improves speed/readability/etc.)
• Bug fix (non-breaking change which fixes an issue)
• Chore (any other clean up, maintenance, etc.)

Key checklist

• No style issues: $ nox -s linter [-- format]
• Code is free of misspellings: $ nox -s codespell [-- write]
• All tests pass: $ nox -s tests
• Badges are updated: $ nox -s badges

Further checks:

• The documentation builds: $ nox -s docs.
• Code is commented, particularly in hard-to-understand areas.
• Tests are added that prove fix is effective or that feature works.