Brusselator and MOL_Interface2 tests stall for 6+ hours in CI

[ChrisRackauckas-Claude]

Description

The Brusselator and MOL_Interface2 test groups consistently stall for 6+ hours in CI, hitting the GitHub Actions timeout limit. This occurs across all Julia versions (1, lts, pre).

Evidence

In PR #507 CI runs:

• Brusselator (all 3 Julia versions): Ran from 11:36 to 17:36 UTC (6 hours) before being cancelled by timeout. All were stuck on step 6 (running tests).
• MOL_Interface2 (all 3 Julia versions): Same behavior - ran for the full 6-hour limit before cancellation.
• In a previous CI run, the same tests ran for 2+ hours before being cancelled by a newer commit push.

Impact

• These stalling tests also cause the Downgrade workflow to timeout, since it runs GROUP="All" which includes both test groups.
• The 6 cancelled jobs prevent achieving green CI.

Current Workaround

Temporarily disabled both test groups:

• Removed from CI matrix in .github/workflows/Tests.yml
• Excluded from GROUP=="All" in test/runtests.jl (can still be run individually with GROUP="Brusselator" or GROUP="MOL_Interface2")

Investigation Needed

• The Brusselator test (test/pde_systems/brusselator_eq.jl) is a 2D reaction-diffusion system that generates a large ODE. It may be a compilation/solving performance regression.
• The MOL_Interface2 test (test/pde_systems/MOLtest2.jl) has many varied PDE system discretizations.
• Need to investigate whether these tests ever completed successfully in recent CI history, and what changed to cause them to stall.