Avoid non-lazy string interpolation and replace @assert with ArgumentError

[devmotion]

Fixes #778:

julia> using ForwardDiff, JET

julia> @test_opt ForwardDiff.gradient(only, [1.0], ForwardDiff.GradientConfig(only, [1.0], ForwardDiff.Chunk{1}()))
Test Passed

julia> @test_opt ForwardDiff.jacobian(identity, [1.0], ForwardDiff.JacobianConfig(identity, [1.0], ForwardDiff.Chunk{1}()))
Test Passed

julia> versioninfo()
Julia Version 1.12.0-rc3
Commit 7522b240144 (2025-09-26 07:42 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 10 × Apple M2 Pro
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, apple-m2)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 6 virtual cores)

(jl_vdSkpK) pkg> st
Status `/private/var/folders/n6/98_7bm0j0hb57zv3l3tj8sxh0000gn/T/jl_vdSkpK/Project.toml`
  [f6369f11] ForwardDiff v1.2.1 `https://github.com/JuliaDiff/ForwardDiff.jl.git#dw/string_assert`
  [c3a54625] JET v0.10.8