Fix Vararg syntax error for Julia 1.12 compatibility

Replace @variables ∫(.., ..) with SymbolicUtils.@syms ∫(::Any, ::Any)::Real

The original syntax @variables ∫(.., ..) caused a TypeError in Julia 1.12:
"TypeError: in Tuple, in non-final parameter, expected Type, got Vararg"

This was preventing package registration for v3.2.0. The new syntax uses
SymbolicUtils.@syms which is compatible with Julia 1.12 and correctly
defines the unevaluated integral symbol as a function taking two arguments.

Fixes package loading error at src/methods/risch/frontend.jl:728

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

src/methods/risch/frontend.jl

@@ -725,7 +725,7 @@ function TowerOfDifferentialFields(terms::Vector{Term})  where
 end
 
 
-@variables ∫(.., ..)
+SymbolicUtils.@syms ∫(::Any, ::Any)::Real
 
 """
     integrate(f, x; kwargs...)