Re-enable downgrade tests after OrdinaryDiffEq updates

[ChrisRackauckas-Claude]

Summary

The downgrade CI tests have been temporarily disabled in the CI workflow due to compatibility issues that need to be resolved with OrdinaryDiffEq updates.

Background

Similar to other SciML packages (e.g., DelayDiffEq.jl), the downgrade tests are failing due to changes in the dependency ecosystem. These tests will need to be re-enabled once the upstream OrdinaryDiffEq updates are complete.

Action Items

• Monitor OrdinaryDiffEq for relevant updates
• Re-enable the downgrade tests by removing if: false from .github/workflows/Downgrade.yml
• Verify tests pass after re-enabling

Related

• See the PR that disabled these tests for context