Re-enable downgrade CI tests

[ChrisRackauckas-Claude]

Summary

The downgrade CI tests have been temporarily disabled in PR #141 due to precompilation failures with downstream dependencies.

Error

When running downgrade tests, the following errors occur:

ERROR: LoadError: UndefVarError: `ispublic` not defined

This originates from ModelingToolkit failing to precompile, which cascades to DataDrivenDiffEq and ultimately SymbolicNumericIntegration.

Related

• PR Fix SymbolicUtils v4.9+ and Symbolics v7.2+ API compatibility #141 disables the downgrade tests

Action Items

• Wait for upstream fixes in ModelingToolkit/DataDrivenDiffEq
• Re-enable downgrade tests by removing if: false from .github/workflows/Downgrade.yml