Import MatrixOperator from SciMLOperators in integrator tests

[ChrisRackauckas-Claude]

Ignore until reviewed by @ChrisRackauckas.

Summary

• Import MatrixOperator directly from its public owner, SciMLOperators, in the Events test.
• Do the same in the Step Limiter test, including its ExponentialRK coverage.
• Keep the production API and dependency compatibility unchanged; SciMLOperators is already a root test extra and target.

Investigation

These isolated test files currently receive MatrixOperator indirectly through SciMLBase's former @reexport using SciMLOperators.

• SciMLBase 7b806c50 (v3.40.0) still makes MatrixOperator visible after using SciMLBase.
• Its immediate child b9c3507a removes the SciMLOperators reexport, so the name is no longer visible through SciMLBase.
• SciMLOperators itself reports MatrixOperator public and exported, has its constructor docstring, and renders it in docs/src/premade_operators.md.

This is therefore an owner-import cleanup and forward-stack runtime fix. It is not a failure with the currently registered stack: master itself first hits the independent SciMLBase floor conflict tracked by #4066, and master plus #4066 resolves SciMLBase 3.39.1, where both test files still pass through the old reexport.

With the owner-clean local SciMLBase 3.40.1 source stack, the unmodified Events test errors with UndefVarError: MatrixOperator not defined. The direct imports remove that dependency on a transitive binding.

Local validation

• Full repository: Runic --check . — passed.
• Owner-clean SciMLBase 3.40.1 validation stack (current master + exact Restore SciMLBase 3.39 compatibility for owner imports #4066 compatibility patch + Keep callback controller resets opt-in #4075 callback fix, with only this PR's two imports uncommitted):
  • Official Integrators_I harness, Events: 57/57 passed.
  • Step Limiter file in the Pkg.test-generated environment:
    • main limiter tests: 149/149
    • solve-level limiter tests: 3/3
    • ExponentialRK limiter tests: 16/16
    • deprecated-field tests: 3/3
    • stage-limiter trait tests: 80/80
• Registered SciMLBase 3.39.1 validation:
  • Events: 57/57
  • official Step Limiter safe testset: 251/251

The #4066 and #4075 commits were used only in disposable validation worktrees and are absent from this branch.

[ChrisRackauckas-Claude]

One-hour terminal CI audit for exact head e8e69e2683: 8 checks succeeded, 4 were skipped, and 115 failed. No job reached a failure in either added MatrixOperator import.

Breakdown:

• 44 root test lanes stopped before tests: 15 in the in-repo [sources] development step and 29 in julia-buildpkg. Representative logs from both paths show the same registered SciMLBase 3.39.1 versus local OrdinaryDiffEqNonlinearSolve = 3.40 conflict fixed by Restore SciMLBase 3.39 compatibility for owner imports #4066.
• all 12 downstream failures were checked individually; every log contains a SciMLBase unsatisfiable requirement before downstream tests.
• documentation stopped at the same SciMLBase resolver boundary.
• 56 sublibrary downgrade jobs plus the root downgrade job stopped inside the downgrade resolver before package tests; this PR changes no dependency or compat metadata.
• Runic reports only the three clean-master DISCO files owned by Run Runic on the disco-optimizations files #4064. The two files changed here, alg_events_tests.jl and step_limiter_test.jl, are explicitly green in that Runic log.

Local source-stack validation remains the evidence for behavior: registered-stack Events 57/57 and Step Limiter 251/251; owner-clean SciMLBase 3.40.1 Events 57/57 and all Step Limiter sections 251/251 total. No resolver workaround or unrelated formatting change is included.