Drop the Newton 1.5 tendon-adapter shim

Newton 1.6.0rc1 provides the ``mujoco:actuator`` custom-frequency API that the
shim backfilled (newton-physics/newton#4017): ``custom_frequency_counts`` and
``custom_frequency_labels`` on the articulation view, and
``custom_frequency_articulation`` on the model. The pin moved to 1.6.0rc1 in
#7445, so ``ensure_newton_custom_frequency_api`` already returned both
arguments unchanged.

The adapter now reads the view and model directly. Removes the shim and its
tests; no other module referenced them.

Author: hujc7

source/isaaclab_newton/changelog.d/jichuanh-drop-newton15-tendon-shim.minor.rst

@@ -0,0 +1,8 @@
+Removed
+^^^^^^^
+
+* Removed the Newton 1.5 compatibility shim for the MuJoCo tendon adapter. Newton 1.6.0rc1
+  provides the ``mujoco:actuator`` custom-frequency view API (newton-physics/newton#4017)
+  directly, so the adapter now reads it from the articulation view and model rather than
+  through a wrapper. No migration is needed: the shim was internal and became a no-op once
+  the Newton pin moved to 1.6.0rc1.

source/isaaclab_newton/isaaclab_newton/physics/_mjwarp_view_compat.py

@@ -34,8 +34,6 @@
 from isaaclab_newton.assets import kernels as shared_kernels
 from isaaclab_newton.assets.articulation import kernels as articulation_kernels
 
-from ._mjwarp_view_compat import ensure_newton_custom_frequency_api
-
 if TYPE_CHECKING:
     from newton import Control, Model
     from newton.selection import ArticulationView
@@ -66,9 +64,7 @@ def create(cls, articulation: Articulation, model: Model) -> MjWarpTendonControl
         Returns:
             The adapter, or None when the articulation's tendons are all passive.
         """
-        # Single compatibility seam: on Newton 1.6 this returns both unchanged. Everything below,
-        # and every other module, is written against 1.6 only.
-        view, model = ensure_newton_custom_frequency_api(articulation.root_view, model)
+        view = articulation.root_view
         mapping = resolve_fixed_tendon_actuator_columns(view, model)
         if mapping is None or mapping[0].size == 0:
             return None