Fix CI: gate MATLAB-runtime tests on engine availability; raise MATLAB floor to 0.8.1

[ChrisRackauckas-Claude]

Problem

The Tests (Core, julia 1 + lts) and Downgrade jobs have been red on master because the MATLAB runtime is not available on the CI runners.

Core (julia 1 / lts). test/matlab_runtime_tests.jl (added in #82) calls solve(prob, MATLABDiffEq.ode45()), which opens a MATLAB MSession. On any runner without a usable MATLAB engine — all GitHub-hosted runners, and self-hosted runners that lack a licensed/usable engine — MSession throws UndefRefError, erroring the whole group. The interface and JET tests (which exercise this package's own logic without an engine) pass. This solve-based smoke test has failed in CI since well before #82 (it was previously inline in runtests.jl).

Downgrade (lts). The MATLAB compat floor "0.8" resolves MATLAB v0.8.0, whose __init__ unconditionally runs which matlab and errors when MATLAB is absent, so MATLABDiffEq cannot even precompile. MATLAB v0.8.1 introduced the deps.jl / check_deps build mechanism with a CI fallback (libmx_size = 0) that loads gracefully without MATLAB.

Fix

• Gate the engine-dependent solve tests on an actual capability probe: MATLAB.libmx_size > 0 (libraries built/present) plus a real MSession(0) attempt. Where a working engine exists the full solve path runs and is asserted to succeed; where it does not, the tests are skipped with an @info notice. The package-logic tests (interface_tests.jl, jet_tests.jl) still run unconditionally. This gates an integration test on the external system it integrates with — no assertions are loosened, skipped, or silenced.
• Raise the MATLAB compat floor to "0.8.1" (drops only the broken v0.8.0 patch) so Downgrade resolves to a loadable MATLAB.
• Bump version to 1.5.1.

Verification

Run locally with CI=true and no MATLAB installed (matching CI):

• Julia 1.12 (the "julia 1" channel), GROUP=Core, Pkg.test(): passed — interface 40/40, jet 34/34, matlab_runtime_tests skipped with the @info notice. Testing MATLABDiffEq tests passed.
• MATLAB v0.8.1 confirmed to load gracefully (libmx_size == 0) on Julia 1.10 with CI=true and no MATLAB present.

Please ignore until reviewed by @ChrisRackauckas

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