Move SBDF methods to IMEX/Split solver group

SBDF methods are SplitODEProblem solvers, not standard ODE solvers.
Moved SBDF(order=2), SBDF2(), SBDF3(), and SBDF4() to the IMEX solver
group where they will be tested with the appropriate SplitODEProblem.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

lib/OrdinaryDiffEqBDF/test/allocation_tests.jl

@@ -39,11 +39,10 @@ Currently, many BDF solvers are allocating and marked with @test_broken.
 
     # Test all exported BDF solvers for allocation-free behavior
     # Standard ODE BDF methods
-    bdf_solvers = [ABDF2(), QNDF1(), QBDF1(), QNDF2(), QBDF2(), QNDF(), QBDF(), FBDF(),
-                   SBDF(order=2), SBDF2(), SBDF3(), SBDF4(), MEBDF2()]
+    bdf_solvers = [ABDF2(), QNDF1(), QBDF1(), QNDF2(), QBDF2(), QNDF(), QBDF(), FBDF(), MEBDF2()]
 
-    # IMEX methods need SplitODEProblem
-    imex_solvers = [IMEXEuler(), IMEXEulerARK()]
+    # IMEX/Split methods need SplitODEProblem
+    imex_solvers = [SBDF(order=2), SBDF2(), SBDF3(), SBDF4(), IMEXEuler(), IMEXEulerARK()]
 
     # DAE methods need DAEProblem
     dae_solvers = [DABDF2(), DImplicitEuler(), DFBDF()]