Fix "Fast tests (core only)" CI: skip tests requiring optional Tasmanian/pyomo dependencies

[Copilot]

The fast CI job (.[dev] install only) was failing with ModuleNotFoundError on 4 tests that exercise optional-dependency code paths — Tasmanian (sparse-grid acquisition) and pyomo (Müller case studies CS2/CS3) — neither of which is installed in that job.

Changes

• tests/unit/test_acquisition.py — added pytest.importorskip("Tasmanian") to the two sparse-grid EI tests that invoke the SPARSE_GRID acquisition path:

  • test_ei_sparse_is_deterministic_and_nonnegative
  • test_ei_sparse_ldl_fallback_for_non_positive_definite_covariance

• tests/unit/test_calibration_problem.py — added pytest.importorskip("pyomo") inside the parametrized test_get_case_study_returns_valid_problem for cs in (2, 3), which construct CSMuller and call into __solve_pyomo_Muller_min at init time.

# test_acquisition.py
def test_ei_sparse_is_deterministic_and_nonnegative():
    pytest.importorskip("Tasmanian")
    ...

# test_calibration_problem.py
def test_get_case_study_returns_valid_problem(cs):
    if cs in (2, 3):
        pytest.importorskip("pyomo")
    ...

Both modules are already lazy-imported in production code; only the test side was missing the guard.