refactor(autojac): Factorize jacobian dict creation

[PierreQuinton]

This is mostly to test opencode, but I think it is valuable. I asked it to review the autojac package, it suggested that (with a little bit of back and forth between it and me), I like it, but we don't have to accept it.

[PierreQuinton]

/opencode:Plan Can you review these changes? Anything to report?

[opencode-agent]

This is a solid refactoring that cleanly deduplicates code. Here's my review:

Summary

The PR extracts 30+ lines of duplicated jacobian dict creation logic from both _backward.py and _jac.py into a new create_jac_dict function in _utils.py.

Changes Reviewed

_utils.py (new code, lines 106-156):

• New create_jac_dict function that handles both the identity case (when opt_jacobians=None) and the validation case
• New check_consistent_first_dimension helper function for validating that all Jacobians have the same first dimension
• Clean docstrings following project conventions
• Proper type hints throughout

_backward.py and _jac.py:

• Removed duplicated inline logic (~30 lines each)
• Now import and call create_jac_dict instead
• Simpler, more maintainable code

Tests (test_backward.py, test_jac.py):

• Updated to import and use create_jac_dict in the _create_transform tests
• Tests properly validate the extracted function works correctly

Assessment

✅ No issues found - This is a clean deduplication that:

• Preserves all existing behavior
• Improves maintainability
• Has proper validation
• Includes good test coverage
• Follows the project's code style

The refactoring is ready to merge.

opencode session  |  github run

[ValerianRey]

LGTM, nice catch! (CI will break until #611 is merged)