Re-enable downgrade tests once JET compatibility is resolved

[ChrisRackauckas-Claude]

Summary

The downgrade tests have been temporarily disabled in #[PR to be created] due to JET version compatibility issues with the downgraded package versions.

Background

JET.jl has strict version requirements that weren't fully captured in the registry:

• JET 0.9.x requires Julia 1.10 only
• JET 0.9.19+ requires Julia 1.11 only
• JET 0.10.x and 0.11.x require Julia 1.12 only

When running downgrade tests, the downgraded package versions may not be compatible with the JET version requirements, causing test failures.

Solution

The downgrade tests should be re-enabled once:

1. JET's version compatibility is properly reflected in the registry
2. The dynamic JET loading approach in runtests.jl handles version mismatches gracefully
3. Or JET tests are completely excluded from downgrade test runs via the GROUP environment variable

Related

• PR Add JET.jl static analysis tests #209 - Original JET tests addition
• Similar issue in DelayDiffEq.jl: Disable downgrade tests pending OrdinaryDiffEq updates DelayDiffEq.jl#357