Downgrade sublibrary tests disabled

[ChrisRackauckas-Claude]

The downgrade CI workflows are currently disabled (if: false):

Downgrade.yml (Core):

• Disabled pending verification that core downgrade works with Optim v2 requirement in OptimizationOptimJL.

DowngradeSublibraries.yml:

• OptimizationOptimJL: Now requires Optim v2 only (compat set to Optim = "2"). Downgrade should work since lowest version is Optim 2.0.0, but needs verification.
• OptimizationSciPy: PythonCall initialization error with Julia 1.10
• OptimizationQuadDIRECT: QuadDIRECT algorithm assertion error

These tests should be re-enabled once the upstream issues are resolved and verified.

Related: PR #1155 (Optim v2 support for OptimizationOptimJL)