Fix startswith path bug; add USD->Newton and test cleanup

- Fix _scope_custom_frequencies: use root_path.rstrip("/")+"/": prefix
  instead of bare startswith(root_path) to prevent sibling paths like
  /robot_a_v2/tendon from falsely matching a filter scoped to /robot_a
- Add TestUsdTendonParsing: real in-memory USD stage with MjcTendon prim,
  verifies stiffness authored in USD lands in proto builder (no Isaac Sim
  required), and that plain builder without register_custom_attributes
  picks up no tendon data
- Add test_filter_rejects_sibling_path_prefix_match covering the bug above
- Trim test suite from 24 to 12: remove Newton-API-only tests
  (TestMainBuilderHasNoMjcFrequencies, TestProtoBuilderHasMjcFrequencies,
  importable/instantiable import checks) and redundant coverage
  (single_world, span_all_worlds, newton_filter, plain_builder,
  both_frequencies_patched, noop_on_plain, heterogeneous_plan)

Author: Horde

source/isaaclab_newton/isaaclab_newton/cloner/newton_replicate.py

@@ -43,8 +43,9 @@ def _scope_custom_frequencies(builder: ModelBuilder, root_path: str) -> None:
         if freq.usd_prim_filter is None:
             continue
         orig = freq.usd_prim_filter
-        freq.usd_prim_filter = lambda prim, ctx, _orig=orig, _path=root_path: (
-            str(prim.GetPath()).startswith(_path) and _orig(prim, ctx)
+        _prefix = root_path.rstrip("/") + "/"
+        freq.usd_prim_filter = lambda prim, ctx, _orig=orig, _prefix=_prefix: (
+            str(prim.GetPath()).startswith(_prefix) and _orig(prim, ctx)
         )