Add copy method for WOperator

[ChrisRackauckas-Claude]

Summary

This PR adds a missing Base.copy method for the WOperator type, which was causing MethodError when trying to copy these objects.

Problem

The WOperator type in OrdinaryDiffEqDifferentiation was missing a copy overload, leading to:

MethodError(copy, (OrdinaryDiffEqDifferentiation.WOperator{...}))

This issue was discovered during CI testing related to DataStructures v0.19 compatibility.

Solution

• Implemented Base.copy(W::WOperator{IIP}) that properly handles the complex internal structure
• The method reconstructs the operator using the public constructor while preserving internal computed state
• Ensures independence between original and copied objects
• Handles different scenarios for size inference when creating the temporary u vector

Implementation Details

The copy method:

1. Infers the appropriate size for the u parameter needed by the constructor
2. Creates a new WOperator using the public constructor with original fields
3. Manually copies internal computed fields (_func_cache and _concrete_form)
4. Returns a fully functional, independent copy

Testing

• ✅ Basic copy functionality works correctly
• ✅ Copied objects are independent (modifications don't affect each other)
• ✅ Copied WOperator maintains full functionality
• ✅ Integration tests with stiff ODE solvers (Rosenbrock23, KenCarp4) still pass
• ✅ No regression in existing ODE solving capabilities

Test plan

• Verify copy method works with different mass matrix types (UniformScaling, Matrix)
• Confirm copied objects are independent
• Test that copied WOperator can perform matrix operations
• Verify no regression in ODE solver functionality
• Run comprehensive tests with realistic scenarios

🤖 Generated with Claude Code