Fix master CI: replace removed ODEFunction syms kwarg with SymbolCache via sys

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Problem

master is red because DifferenceEquations itself fails to precompile:

ERROR: LoadError: MethodError: no method matching (SciMLBase.ODEFunction{false, SciMLBase.AutoSpecialize})(::DifferenceEquations.var"#10#11"; syms::Nothing)
    @ ~/work/DifferenceEquations.jl/DifferenceEquations.jl/src/precompilation.jl:19

(failing run: 25613839863)

The syms keyword on ODEFunction was removed in newer SciMLBase versions; symbol metadata now flows through a SymbolCache attached via the sys keyword (powered by SymbolicIndexingInterface). LinearStateSpaceProblem and QuadraticStateSpaceProblem were still constructing their placeholder ODEFunction with syms = syms, which fails as soon as the deps resolve to a SymbolicIndexingInterface-era SciMLBase.

Fix

• In LinearStateSpaceProblem / QuadraticStateSpaceProblem, keep the user-facing syms = ... keyword for backwards compatibility, but translate to sys = syms === nothing ? nothing : SymbolCache(collect(syms)) when constructing the placeholder ODEFunction.
• Add SymbolicIndexingInterface as an explicit dep with compat = "0.3" and import SymbolCache explicitly (so ExplicitImports checks still pass).

Verification

• Pkg.precompile() now succeeds on Julia 1.10 and 1.12.
• The DataFrame test from test/sciml_interfaces.jl still produces the expected column names:

  prob = LinearStateSpaceProblem(...; syms = (:a, :b))
  df = DataFrame(solve(prob))
  propertynames(df) == [:timestamp, :a, :b]  # passes

• The :plotting given noise testset (also from sciml_interfaces.jl) now runs and passes.

Other downstream test failures observed locally (MethodError: no method matching size(::ZeroTangent) in RecursiveArrayToolsZygoteExt, ensemble plotting BoundsError) are independent issues unmasked by precompilation working again — they'll be addressed in follow-up PRs to keep this one focused on unbreaking master.

Test plan

• CI passes on Julia 1, 1.10 (lts), and pre

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