Support pinned flex vertices with bending

The flexcomp compiler previously rejected pins on dim-2 flexes with
bending (elastic2d bend/both). Allow them: mj_flexPassiveBend treats a
pinned vertex (body without 3 free slide dofs) as static -- zero
velocity, and no bending force applied to it (the reaction is carried
by the pin) -- while its position still enters every neighbor's
bending force, which is exactly what the pin constrains.

PiperOrigin-RevId: 944410969
Change-Id: Ib0a69c8d5fb6f64d3e2a76af3b6b2c7be1898191

test/engine/engine_passive_test.cc

@@ -965,5 +965,46 @@ TEST_F(ElasticityTest, InterpBendingRigidRotationInvariance) {
   }
 }
 
+
+// verify that a pinned vertex (on a static body) gets zero bending force
+// while its free neighbors get nonzero bending force
+TEST_F(ElasticityTest, PinnedVertexBendingForce) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+  <worldbody>
+    <body name="parent">
+      <flexcomp name="test" type="grid" count="3 3 1" spacing="1 1 1"
+                radius="0.01" dim="2">
+        <elasticity young="1" poisson="0" thickness="1" elastic2d="bend"/>
+        <pin id="0"/>
+      </flexcomp>
+    </body>
+  </worldbody>
+  </mujoco>
+  )";
+
+  char error[1024] = {0};
+  MjModelPtr m = LoadModelFromString(xml, error, sizeof(error));
+  ASSERT_THAT(m.get(), testing::NotNull()) << error;
+  MjDataPtr d = MakeData(m);
+
+  // displace a free vertex out of plane to create bending
+  d->qpos[3] = 0.1;
+  mj_forward(m.get(), d.get());
+
+  // vertex 0 is pinned to the world body (body 0), which has 0 dofs,
+  // so no bending force is written for it — the reaction is absorbed by the pin
+
+  // verify that at least some free vertices have nonzero spring force
+  bool has_nonzero = false;
+  for (int i = 0; i < m->nv; i++) {
+    if (d->qfrc_spring[i] != 0) {
+      has_nonzero = true;
+      break;
+    }
+  }
+  EXPECT_TRUE(has_nonzero) << "bending should produce nonzero spring forces";
+}
+
 }  // namespace
 }  // namespace mujoco