Fix deprecated DataStructures.jl functions for v0.19 compatibility

- Replace enqueue! with push!
- Replace dequeue! with popfirst!
- Replace IntDisjointSets with IntDisjointSet
- Import IntDisjointSet to fix compilation

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

src/ModelingToolkit.jl

@@ -29,6 +29,7 @@ using LinearAlgebra, SparseArrays
 using InteractiveUtils
 using JumpProcesses
 using DataStructures
+import DataStructures: IntDisjointSet
 using Base.Threads
 using Latexify, Unitful, ArrayInterface
 using Setfield, ConstructionBase

src/systems/alias_elimination.jl

@@ -426,20 +426,20 @@ function topsort_equations(eqs, unknowns; check = true)
 
     q = Queue{Int}(neqs)
     for (i, d) in enumerate(degrees)
-        d == 0 && enqueue!(q, i)
+        d == 0 && push!(q, i)
     end
 
     idx = 0
     ordered_eqs = similar(eqs, 0)
     sizehint!(ordered_eqs, neqs)
     while !isempty(q)
-        𝑠eq = dequeue!(q)
+        𝑠eq = popfirst!(q)
         idx += 1
         push!(ordered_eqs, eqs[𝑠eq])
         var = assigns[𝑠eq]
         for 𝑑eq in 𝑑neighbors(graph, var)
             degree = degrees[𝑑eq] = degrees[𝑑eq] - 1
-            degree == 0 && enqueue!(q, 𝑑eq)
+            degree == 0 && push!(q, 𝑑eq)
         end
     end
 

src/systems/connectiongraph.jl

@@ -452,7 +452,7 @@ function connectionsets(graph::ConnectionGraph)
     invmap = graph.invmap
 
     # union all of the hyperedges
-    disjoint_sets = IntDisjointSets(length(invmap))
+    disjoint_sets = IntDisjointSet(length(invmap))
     for edge_i in 𝑠vertices(bigraph)
         hyperedge = 𝑠neighbors(bigraph, edge_i)
         isempty(hyperedge) && continue