Canonicalize test units to @safetestset

[ChrisRackauckas-Claude]

Wraps each independent top-level test unit in @safetestset so it runs in its own module, matching OrdinaryDiffEq's canonical structure (isolation between units + world-age safety). Behavior-preserving: the same units run under the same GROUP dispatch with the same assertions — only the unit wrapper changes.

Changes

• test/runtests.jl: the 8 Core units @testset "X" begin include("x.jl") end become @safetestset "X" begin include("x.jl") end. Each included file (algorithm_tests.jl, saving_tests.jl, mass_matrix_tests.jl, jac_tests.jl, callbacks.jl, initialization_tests.jl, mtk_initialization_tests.jl, explicit_imports.jl) is already self-contained with its own using/import lines, and the include() form runs statements sequentially so imports (and package macros like ModelingToolkit's @variables/@mtkcompile) resolve correctly inside the fresh module.
• test/qa/qa.jl: the Aqua and JET units become @safetestset, each carrying its own using ODEInterfaceDiffEq, <Tool>, Test. The existing @test_broken lines (tracked in QA: Aqua/JET findings marked @test_broken pending fix #105) are preserved verbatim. The outer @testset "Quality Assurance" grouping wrapper in runtests.jl is left as-is.
• Deps: added SafeTestsets to the main Project.toml [extras] + [targets].test + [compat] SafeTestsets = "0.1, 1", and to test/qa/Project.toml (the QA group activates its own env).

The GROUP-dispatch ladder, run path, and all assertions are unchanged. Nested grouping @testsets inside a unit (e.g. the per-iip/per-alg testsets in the initialization files) stay plain @testset — the unit-level @safetestset already isolates them.

Verification

GROUP=Core run locally via Pkg.test (Julia 1.11) — passed:

Explicit Imports |    2      2
Algorithms       |     0           (exercises solve() calls, no @test — unchanged)
Saving           |    3      3
Mass Matrix      |    4      4
Jacobian Tests   |    1      1
Callback Tests   |    2      2
Initialization Tests     |   13     13
MTK Initialization Tests |   55     55
     Testing ODEInterfaceDiffEq tests passed

Ignore until reviewed by @ChrisRackauckas.