Fix NodeContext's handling of namespaced singleton method (#2385)

When handling namespaced singleton method, the `handle_nesting_nodes`
method should split the current last nesting before creating the singleton class
nesting with it.

Failing to do so would let the TypeInferrer infer the wrong type for the
singleton method's receiver.

lib/ruby_lsp/node_context.rb

@@ -89,12 +89,12 @@ def handle_nesting_nodes(nodes)
         when Prism::ClassNode, Prism::ModuleNode
           nesting << node.constant_path.slice
         when Prism::SingletonClassNode
-          nesting << "<Class:#{nesting.last}>"
+          nesting << "<Class:#{nesting.flat_map { |n| n.split("::") }.last}>"
         when Prism::DefNode
           surrounding_method = node.name.to_s
           next unless node.receiver.is_a?(Prism::SelfNode)
 
-          nesting << "<Class:#{nesting.last}>"
+          nesting << "<Class:#{nesting.flat_map { |n| n.split("::") }.last}>"
         end
       end
 

test/type_inferrer_test.rb

@@ -138,6 +138,19 @@ class << self
       assert_equal("Foo::<Class:Foo>::<Class:<Class:Foo>>", @type_inferrer.infer_receiver_type(node_context).name)
     end
 
+    def test_infer_receiver_type_in_namespaced_singleton_method
+      node_context = index_and_locate(<<~RUBY, { line: 2, character: 4 })
+        class Foo::Bar
+          def self.foo
+            bar
+          end
+        end
+      RUBY
+
+      result = @type_inferrer.infer_receiver_type(node_context).name
+      assert_equal("Foo::Bar::<Class:Bar>", result)
+    end
+
     def test_infer_receiver_type_instance_variables_in_singleton_block_method
       node_context = index_and_locate(<<~RUBY, { line: 3, character: 6 })
         class Foo