Fix stale downstream test groups

[ChrisRackauckas-Claude]

Summary

• select DataDrivenDiffEq's existing Core group instead of the nonexistent Downstream group
• replace MethodOfLines' nonexistent Interface group with its two current interface lanes, MOL_Interface1 and MOL_Interface2

This is stacked on #4759 so the downstream workflow also propagates resolver and test failures. The extra #4755/#4759 commits will disappear from this diff as those prerequisites merge.

Ignore this PR until reviewed by @ChrisRackauckas.

Local verification

Each corrected target was run with this ModelingToolkit branch developed into the downstream package:

• DataDrivenDiffEq/Core: passed; substantive Basis, DataDrivenProblem, DataDrivenSolution, Utilities, and CommonSolve summaries
• MethodOfLines/MOL_Interface1: passed after 18m58s; the lane executes its PDE construction and solve smoke cases but contains no explicit @test assertions
• MethodOfLines/MOL_Interface2: passed after 11m51s; 4 passed and 8 pre-existing broken assertions

Final stacked branch checks:

• actionlint -color
• /home/crackauc/.juliaup/bin/julialauncher +1.12 --startup-file=no -m Runic --check .
• git diff --check agent/propagate-workflow-failures..HEAD

[ChrisRackauckas-Claude]

The SBMLToolkit.jl/All/1 failure in job 87321454608 is not caused by the two stale-group corrections in this PR. This branch is stacked on #4759, which removes the resolver-error catch and therefore exposes an existing incompatibility: SBMLToolkit pins SBMLToolkitTestSuite 0.0.5, while the resolved OrdinaryDiffEq 7 stack requires TestSuite 1.x. The default branch's corresponding job appears green only because that resolver error is still swallowed there. A separate clean-source compatibility investigation is queued; this PR remains limited to DataDrivenDiffEq and MethodOfLines group selection.