Fix GPU test: convert(AbstractArray, va) is now identity

Since AbstractVectorOfArray <: AbstractArray, convert(AbstractArray, va)
returns va itself. Use stack(va.u) to get a dense CuArray instead.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

test/gpu/vectorofarray_gpu.jl

@@ -36,8 +36,8 @@ function f(p)
 end
 Zygote.gradient(f, p)
 
-# Check conversion preserves device
-va_cu = convert(AbstractArray, va)
+# Check conversion to dense GPU array
+va_cu = stack(va.u)
 
 @test va_cu isa CuArray
 @test size(va_cu) == size(x)