Add missing butterfly.jl test to runtests.jl with version check

The butterfly.jl test file was present in the test directory but was not
being run as part of the test suite. This test file validates the
ButterflyFactorization algorithm with random matrices and Wilkinson
matrices.

The test is conditionally included only when RecursiveFactorization
version 0.2.26 or higher is available, as the butterfly factorization
functionality requires that version.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ChrisRackauckas

test/runtests.jl

@@ -19,6 +19,16 @@ if GROUP == "All" || GROUP == "Core"
     @time @safetestset "Traits" include("traits.jl")
     @time @safetestset "Verbosity" include("verbosity.jl")
     @time @safetestset "BandedMatrices" include("banded.jl")
+    # Butterfly test requires RecursiveFactorization 0.2.26+
+    if HAS_EXTENSIONS
+        try
+            import RecursiveFactorization
+            if pkgversion(RecursiveFactorization) >= v"0.2.26"
+                @time @safetestset "Butterfly Factorization" include("butterfly.jl")
+            end
+        catch
+        end
+    end
     @time @safetestset "Mixed Precision" include("test_mixed_precision.jl")
 end