Cover degree-1 (linear) BSplineInterpolation in thread-safety tests

A degree-1 B-spline goes through the same _interpolate method as higher
degrees (dispatch is on type, not degree), so it was equally subject to the
shared-buffer race; add it explicitly to document the coverage.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 2 people

test/Methods/thread_safety_tests.jl

@@ -19,6 +19,8 @@ function thread_safety_interpolants()
     ]            # 2 × length(t): last axis indexes t
     return [
         ("BSplineInterpolation", BSplineInterpolation(u, t, 3, :Uniform, :Average)),
+        # degree 1 == "linear" B-spline: same `_interpolate` method, so equally affected
+        ("BSplineInterpolation (degree 1)", BSplineInterpolation(u, t, 1, :Uniform, :Average)),
         ("BSplineApprox", BSplineApprox(u, t, 3, 4, :Uniform, :Average)),
         ("BSplineInterpolation (array)", BSplineInterpolation(umat, t, 3, :Uniform, :Average)),
         ("BSplineApprox (array)", BSplineApprox(umat, t, 3, 4, :Uniform, :Average)),