Add signature to test flow

Author: Taucher2003

gem/spec/support/protobuf_factories.rb

@@ -8,7 +8,8 @@ def build_flow(node_functions:, starting_node_id: 1)
       type: 'test',
       starting_node_id: starting_node_id,
       node_functions: node_functions,
-      project_slug: 'test'
+      project_slug: 'test',
+      signature: '(): void'
     )
   end
 

gem/spec/triangulum/validation_spec.rb

@@ -100,6 +100,7 @@
       result = described_class.new(flow, functions, data_types).validate
 
       expect(result.valid?).to be true
+      expect(result.diagnostics).to be_empty
     end
 
     it 'returns diagnostics with node_id and parameter_index' do