Alias derivative_discontinuity field to u_modified for SciMLBase v3

[ChrisRackauckas-Claude]

Problem

On the latest dependency resolution (SciMLBase v3, DiffEqBase that reads the new field name), Pkg.test() fails in the Callback Tests with:

type ODEInterfaceIntegrator has no field derivative_discontinuity
  [1] getproperty @ src/integrator_utils.jl:94
  [2] initialize! @ DiffEqBase/src/callbacks.jl:25
  ...
  [13] test/callbacks.jl:23  (ContinuousCallback solve with dopri5)

Root cause

SciMLBase v3 renamed the u_modified! API to derivative_discontinuity!, and DiffEqBase's callback machinery now reads and writes the integrator field derivative_discontinuity directly (callbacks.jl initialize!/finalize!/apply_discrete_callback!/apply_callback! all do integrator.derivative_discontinuity and integrator.derivative_discontinuity = ...).

ODEInterfaceIntegrator only has a u_modified field. The repo already added a derivative_discontinuity! method shim, but that does not help when the callback code touches the field. The getproperty override (which already aliases :dt) had no entry for :derivative_discontinuity, and there was no setproperty! override at all.

Fix

Alias derivative_discontinuity to the real u_modified field in both getproperty and a new setproperty!. The aliasing is version-agnostic: on SciMLBase v2 the field is accessed as u_modified directly (unaffected), and on v3 the derivative_discontinuity accesses now map to u_modified. The new setproperty! preserves the implicit convert(fieldtype(...), x) that Julia's default setproperty! performs, so the other (numeric) field assignments in handle_callbacks!/apply_callback! still convert correctly.

Only src/integrator_utils.jl is touched. No changes to Project.toml or CI config.

Testing

Full suite run locally at latest deps (SciMLBase v3.18.0), Julia 1.10:

Before:

Test Summary:  | Error  Total  Time
Callback Tests |     1      1  1.9s
ERROR: Package ODEInterfaceDiffEq errored during testing

After:

Test Summary:  | Pass  Total  Time
Callback Tests |    2      2  1.8s
...
     Testing ODEInterfaceDiffEq tests passed

Full suite run locally at latest deps: PASS.

Ignore until reviewed by @ChrisRackauckas.

🤖 Generated with Claude Code