What changed

Widen the DataInterpolations compatibility entry to include version 9 and exercise the version-correct BSpline constructor in the existing interpolation test. This lets current ModelingToolkit/ModelingToolkitBase downstream tests resolve the shared FindFirstFunctions 3 dependency while retaining the DataInterpolations 8 test path.

Fixes #498.

Root cause

SciMLBase 3.35 and later require FindFirstFunctions 3, while DataInterpolations 8 is restricted to FindFirstFunctions 1.3–1.8. DataInterpolations 9 moved to FindFirstFunctions 3, but it also removed the old pVecType positional BSpline constructor argument, so widening compat alone makes test/sources.jl error.

Verification

Failing before, from MTSL dd512368441fc4684b2a6f6a011447dbe5cd80c8 against clean ModelingToolkit d7f3ad523c00b4150281f259d1e97618fba79982:

DataInterpolations 8 restricts FindFirstFunctions to 1.3-1.8
SciMLBase 3.44 restricts FindFirstFunctions to 3
ERROR: Unsatisfiable requirements detected for package FindFirstFunctions

With only the compat widened, the existing Core test discriminated the constructor change:

Core/sources.jl | 63 passed, 1 errored, 1 broken
MethodError: no method matching BSplineInterpolation(..., ::Int64, ::Symbol, ::Symbol)

With this patch:

GROUP=Core julia +1.12.6 --startup-file=no --project=. -e 'using Pkg; Pkg.develop(path="../ModelingToolkit.jl/lib/ModelingToolkitBase"); Pkg.develop(path="../ModelingToolkit.jl"); Pkg.update(); Pkg.test(; coverage=false)'

resolved DataInterpolations 9.2.0, FindFirstFunctions 3.2.1, and SciMLBase 3.44.0, then exited 0:

Core/sources.jl | 64 passed, 1 broken, 65 total
Testing ModelingToolkitStandardLibrary tests passed

Additional local checks:

DataInterpolations 8.10.0 standalone BSpline constructor: pass
GROUP=QA julia +release --project=. -e 'using Pkg; Pkg.test(; coverage=false)'
QA/qa.jl | 20 passed, 20 total
Runic --check test/sources.jl: pass
git diff --check: pass
typos over the focused diff: pass

No docs build was run because this changes no public API or documentation.

Please ignore this draft until it has been reviewed by @ChrisRackauckas.