fix CI: isdefined -> isassigned and add Dependabot (#120)

* fix: use isassigned for dijkstra_states slot checks

Julia 1.11+ changed isdefined(::Vector, i) to only check bounds, not
element initialization (Vectors now back onto Memory{T}). Use isassigned
to preserve the original "is this slot populated?" semantics. Without
this, osm_subgraph populated osg.dijkstra_states even when g had all
undef entries, causing UndefRefError when comparing in tests.

* chore: add dependabot for github-actions

Weekly grouped updates so all action bumps land in one PR.

Author: asinghvi17

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      github-actions:
+        patterns:
+          - "*"

src/subgraph.jl

@@ -41,7 +41,7 @@ function osm_subgraph(g::OSMGraph{U, T, W},
     add_graph!(osg, get_graph_type(g))
     add_node_tags!(osg)
 
-    if isdefined(g.dijkstra_states, 1)
+    if !isnothing(g.dijkstra_states) && isassigned(g.dijkstra_states, 1)
         add_dijkstra_states!(osg)
     else
         osg.dijkstra_states = Vector{Vector{U}}(undef, length(osg.nodes))

test/subgraph.jl

@@ -13,8 +13,8 @@ graphs = [
         @test sg.ways == g.ways
         @test sg.restrictions == g.restrictions
         @test sg.weight_type == g.weight_type
-        @test isdefined(sg.dijkstra_states, 1) == isdefined(g.dijkstra_states, 1)
-        if isdefined(g.dijkstra_states, 1)
+        @test isassigned(sg.dijkstra_states, 1) == isassigned(g.dijkstra_states, 1)
+        if isassigned(g.dijkstra_states, 1)
             @test sg.dijkstra_states == g.dijkstra_states
         end
         @test isdefined(sg.kdtree, 1) == isdefined(g.kdtree, 1)