Fix issparse for AbstractVectorOfArray

[ChrisRackauckas-Claude]

Summary

Fixes #486

This PR adds the issparse method for AbstractVectorOfArray, resolving a MethodError that occurred when issparse(::SubArray) tried to call issparse on its parent array.

Changes

1. Added issparse method in RecursiveArrayToolsSparseArraysExt.jl

   • Returns false for AbstractVectorOfArray since it is not a sparse array type
   • Placed in the SparseArrays extension to keep sparse-related functionality together

2. Added comprehensive tests in test/interface_tests.jl

   • Test issparse returns false for VectorOfArray
   • Test issparse returns false for DiffEqArray
   • Test the original issue with SubArray views
   • Test nested VectorOfArray structures

Test Results

All tests pass successfully. The fix resolves the error described in the issue:

julia> U = mapreduce(u -> view(u, :, :), hcat, us)
# Previously: MethodError: no method matching issparse(::ODESolution{...})
# Now: Works correctly, issparse returns false

Checklist

• Added implementation in appropriate extension file
• Added comprehensive tests
• All tests pass locally (129 tests passed)
• Code formatted with JuliaFormatter SciMLStyle

🤖 Generated with Claude Code

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