Problem

Master CI is red on all three tests / Core variants (Julia lts/1/pre) and the Downgrade job. All fail identically:

Core/dde_tests.jl: Error During Test
  Got exception outside of a @test
  LoadError: UndefVarError: `BFGS` not defined
  @ test/dde_tests.jl:42

Root cause

test/dde_tests.jl used BFGS and Optimization without importing the packages that provide them. It only did using DiffEqParamEstim, OptimizationNLopt.

Before the SciMLTesting v1.2 migration (#308), the old runtests.jl included every test file into the same Main module. optim_test.jl's using Optim/OptimizationOptimJL therefore leaked BFGS into shared scope, so the missing import in dde_tests.jl went unnoticed. After #308, each file runs in its own isolated @safetestset module, so the latent missing import surfaces as UndefVarError.

Fix

1. using DiffEqParamEstim, Optimization, OptimizationOptimJL (both already test deps via [extras]/[targets]) so BFGS/Optimization resolve.
2. Drop the dead, unused opt = Opt(:GN_ESCH, 1) line (it referenced Opt from the now-removed OptimizationNLopt import and was never used — the solve uses BFGS()).
3. Switch the loss AD from AutoZygote() to AutoForwardDiff(). Fixing the import revealed a second latent failure: AutoZygote() gradient preparation for this DDE loss errors with MethodError: no method matching _prepare_pullback_aux in DifferentiationInterface 0.7.18. The sibling nlopt_test.jl already documents this exact issue (#zygote behaves weirdly here) and uses AutoForwardDiff(). With ForwardDiff the test actually runs and asserts the real result.

No test was skipped, broken, or loosened — the @test res.u[1] ≈ 0.5 atol = 5.0e-3 assertion is unchanged and genuinely passes.

Local verification

Ran test/dde_tests.jl inside an isolated @safetestset on Julia 1.10 with the exact CI-pinned stack (Optimization 4.8.0, OptimizationBase 2.14.0, OptimizationOptimJL 0.4.5, DifferentiationInterface 0.7.18, Zygote 0.7.10):

Test Summary:      | Pass  Total   Time
DDE Tests isolated |    1      1  20.2s

runic --check test/dde_tests.jl passes (exit 0, no diff).

(The separate Documentation job failure is unrelated — external linkcheck 403/timeout on docs.sciml.ai/Zygote/stable/ and sensitivity.sciml.ai/dev/ — and is not addressed here.)

Please ignore until reviewed by @ChrisRackauckas.

Update (additional fixes)

After fixing dde_tests.jl, the Core/Downgrade jobs surfaced the same latent missing-import bug in two more files, now also fixed in this PR:

• test/likelihood.jl and test/test_on_monte.jl both call the qualified Optimization.OptimizationProblem / Optimization.AutoForwardDiff but only imported a re-exporting backend (OptimizationBBO / OptimizationOptimJL). using OptimizationOptimJL re-exports Optimization's exported symbols but does not bind the Optimization module name into scope, so under v1.2 isolation these errored with UndefVarError: Optimization not defined (likelihood.jl:26 was the first file to abort the Core run; test_on_monte.jl sorts after it and would have failed next). Added Optimization (already a test dep via [extras]/[targets]) to the using line of both.

Local verification (Julia 1.10, the compat floor / CI lts), each file in an isolated @safetestset:

Core/likelihood.jl isolated      | 5 Pass, 0 errors  (was: UndefVarError: Optimization not defined)
Core/test_on_monte.jl isolated   | 1 Pass, 0 errors

Both files are Runic-format clean. No assertions were skipped, broken, or loosened.

Update: the Documentation job is now also fixed here

The Documentation / Build and Deploy red was a pre-existing master failure from the OrdinaryDiffEq v7 / DifferentialEquations v8 ecosystem bump. It is now addressed in this PR (commit docs: migrate examples to current solver/EnsembleSolution API). There were three independent docs-source breakages, all fixed against the current API — no example was skipped, warnonly-ed, or loosened:

1. verbose = false → verbose = None(). OrdinaryDiffEq v7 rejects a Bool for verbose (ArgumentError: Passing a Bool for verbose is no longer supported ... Use DEVerbosity() or a preset like Standard(), None(), etc. from SciMLLogging). None() is the no-output SciMLLogging.AbstractVerbosityPreset (the closest match for the old false). Added using SciMLLogging: None. Affects getting_started.md, generalized_likelihood.md, stochastic_evaluations.md.
2. SDE solvers no longer re-exported. DifferentialEquations v8 was slimmed to {OrdinaryDiffEq, Reexport, SciMLBase} and no longer re-exports SRIW1/SOSRI, so stochastic_evaluations.md hit UndefVarError. Added using StochasticDiffEq: SRIW1, SOSRI (same way the package's own tests import them).
3. EnsembleSolution indexing changed. sim[i] now returns a scalar (flat element index), not the i-th trajectory, so ensemble.md's losses[i](sim[i]) errored with MethodError: no method matching (::L2Loss)(::Float64). Changed to sim.u[i] (the i-th ODESolution).

Added SciMLLogging and StochasticDiffEq to docs/Project.toml [deps]/[compat] (both already in the resolved Manifest as transitive deps; resolution unchanged, DifferentialEquations stays at v8.0.0).

Local verification (Julia 1.12, the failing CI 1 channel): reproduced the original verbose=false ArgumentError, then ran every affected @example block end-to-end after the fix — getting_started recovers a≈1.5 (1-param) and [1.5,1.0,3.0,1.0] (multi-param); ensemble loss is exactly 0.0 on the generating sim and both Fminbox(BFGS())/BFGS() recover [1.5,1.0]; the SDE page's SRIW1/SOSRI solves and None() resolve.