[Workflow] Default manipulation core tasks to Newton (#7065)

Splits the Cabinet and Lift Newton defaults from #6980.\n\n- Defaults
Cabinet, Lift, and Kuka Allegro task configurations to Newton MJWarp.\n-
Includes the shared three-backend core smoke-test matrix and teardown
reliability cleanup.\n- Removes obsolete Play/MJWarp-discovery test
utility logic.\n\nThe shared test commit is intentionally identical to
the other environment shards; rebase/drop it after the first shard
merges.\n\nSupersedes the manipulation portion of #6980.

Author: StafaH

source/isaaclab_tasks/changelog.d/default-newton-manipulation.minor.rst

@@ -0,0 +1,5 @@
+Changed
+^^^^^^^
+
+* **Breaking:** Changed the Cabinet and Lift task families to use Newton MJWarp
+  by default. Pass an explicit physics preset to retain Isaac Sim PhysX.

source/isaaclab_tasks/isaaclab_tasks/core/cabinet/cabinet_env_cfg.py

@@ -102,7 +102,6 @@ class CabinetSimCfg(PresetCfg):
     physx: SimulationCfg = isaacsim_physx.replace(
         physics=PhysxAutoCfg(isaacsim_physx=isaacsim_physx.physics, ovphysx=ovphysx.physics)
     )
-    default: SimulationCfg = isaacsim_physx
     newton_mjwarp: SimulationCfg = SimulationCfg(
         dt=1 / 600,
         render_interval=1,
@@ -125,6 +124,7 @@ class CabinetSimCfg(PresetCfg):
         default_visualizer_cfg=VisualizerCfg(eye=(-2.0, 2.0, 2.0), lookat=(0.8, 0.0, 0.5)),
         physics=NewtonCfg(solver_cfg=KaminoPADMMSolverCfg(max_contacts_per_world=64)),
     )
+    default: SimulationCfg = newton_mjwarp
 
 
 @configclass
@@ -138,9 +138,9 @@ class CabinetDecimationCfg(PresetCfg):
     isaacsim_physx: int = 1
     ovphysx: int = isaacsim_physx
     physx: int = isaacsim_physx
-    default: int = isaacsim_physx
     newton_mjwarp: int = 10
     newton_kamino: int = 10
+    default: int = newton_mjwarp
 
 
 ##

source/isaaclab_tasks/isaaclab_tasks/core/lift/config/kuka_allegro/kuka_allegro_env_cfg.py

@@ -36,7 +36,8 @@ class KukaAllegroPhysicsCfg(lift.PhysicsCfg):
         gpu_found_lost_pairs_capacity=2**26,
     )
     physx = PhysxAutoCfg(isaacsim_physx=isaacsim_physx, ovphysx=ovphysx)
-    default = isaacsim_physx
+    newton_mjwarp = lift.PhysicsCfg().newton_mjwarp
+    default = newton_mjwarp
 
 
 @configclass

source/isaaclab_tasks/isaaclab_tasks/core/lift/lift_env_cfg.py

@@ -510,7 +510,7 @@ class PhysicsCfg(PresetCfg):
         debug_mode=False,
     )
     physx = PhysxAutoCfg(isaacsim_physx=isaacsim_physx)
-    default = isaacsim_physx
+    default = newton_mjwarp
 
 
 @configclass