[planning] Match RobotDiagram default time step to MbP's default

This fixes some crashes when using models that need discrete-time-only
modeling features, such as implicit PD control.

bindings/pydrake/planning/planning_py_robot_diagram.cc

@@ -55,7 +55,8 @@ void DefinePlanningRobotDiagram(py::module m) {
       auto cls = DefineTemplateClassWithDefault<Class>(
           m, "RobotDiagramBuilder", GetPyParam<T>(), cls_doc.doc);
       cls  // BR
-          .def(py::init<double>(), py::arg("time_step") = 0.0, cls_doc.ctor.doc)
+          .def(py::init<double>(), py::arg("time_step") = 0.001,
+              cls_doc.ctor.doc)
           .def("builder",
               overload_cast_explicit<systems::DiagramBuilder<T>&>(
                   &Class::builder),

bindings/pydrake/planning/test/robot_diagram_test.py

@@ -6,11 +6,18 @@
 from pydrake.common.test_utilities import numpy_compare
 from pydrake.geometry import SceneGraph_
 from pydrake.multibody.parsing import Parser
-from pydrake.multibody.plant import MultibodyPlant_
+from pydrake.multibody.plant import MultibodyPlant_, MultibodyPlantConfig
 from pydrake.systems.framework import Context_, DiagramBuilder_
 
 
 class TestRobotDiagram(unittest.TestCase):
+    @numpy_compare.check_all_types
+    def test_robot_diagram_builder_default_time_step(self, T):
+        Class = mut.RobotDiagramBuilder_[T]
+        dut = Class()
+        self.assertEqual(dut.plant().time_step(),
+                         MultibodyPlantConfig().time_step)
+
     @numpy_compare.check_all_types
     def test_robot_diagram_builder(self, T):
         """Tests the full RobotDiagramBuilder API.

planning/robot_diagram_builder.h

@@ -30,7 +30,7 @@ class RobotDiagramBuilder {
   DRAKE_NO_COPY_NO_MOVE_NO_ASSIGN(RobotDiagramBuilder)
 
   /** Constructs with the specified time step for the contained plant. */
-  explicit RobotDiagramBuilder(double time_step = 0.0);
+  explicit RobotDiagramBuilder(double time_step = 0.001);
 
   ~RobotDiagramBuilder();
 

planning/test/robot_diagram_test.cc

@@ -17,6 +17,7 @@ namespace {
 
 using geometry::SceneGraph;
 using multibody::MultibodyPlant;
+using multibody::MultibodyPlantConfig;
 using multibody::Parser;
 using symbolic::Expression;
 using systems::Context;
@@ -32,7 +33,13 @@ std::unique_ptr<RobotDiagramBuilder<double>> MakeSampleDut() {
   return builder;
 }
 
-GTEST_TEST(RobotDiagramBuilderTest, TimeStep) {
+GTEST_TEST(RobotDiagramBuilderTest, TimeStepDefault) {
+  const MultibodyPlantConfig default_plant_config;
+  auto builder = std::make_unique<RobotDiagramBuilder<double>>();
+  EXPECT_EQ(builder->plant().time_step(), default_plant_config.time_step);
+}
+
+GTEST_TEST(RobotDiagramBuilderTest, TimeStepExplicit) {
   const double time_step = 0.01;
   auto builder = std::make_unique<RobotDiagramBuilder<double>>(time_step);
   EXPECT_EQ(builder->plant().time_step(), time_step);