Avoid duplicate MuJoCo schema registration (#7393)

# Description

Isaac Lab exposes the codeless `mjcPhysics` plugin through
`PXR_PLUGINPATH_NAME` before OpenUSD builds its schema registry. The Kit
experiences also enabled `omni.usd.schema.mujoco`, which bundles a
second plugin with the same name and reports the duplicate registration
as an error even though the schema is already available.

This change removes the redundant extension dependency from the GUI and
headless Kit experiences. It keeps the early Isaac Lab schema exposure
needed by kitless MJCF conversion and documents why Kit must not
register a second copy.

No Kit extension replaces `omni.usd.schema.mujoco`: importing `isaaclab`
adds the standalone `mujoco_usd_converter/plugins` directory to
`PXR_PLUGINPATH_NAME`, and the OpenUSD `Plug.Registry` discovers its
codeless schema metadata. This is the same mechanism in Kit and kitless
processes.

No new dependencies are required.

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## Release backport

- [x] <!-- backport-active-release --> Backport this pull request to the
active release branch after it merges into `develop`

## Screenshots

Not applicable; this removes a startup log error.

## Validation

- Live `isaaclab.python.kit` experience: launched successfully with
`headless=True`; `MjcSceneAPI` and the `mjcPhysics` plugin were
available, `omni.usd.schema.mujoco` remained disabled, and the
duplicate-registration error had zero matches
- Fresh kitless probe: `AppLauncher.is_available()` was false while
`MjcSceneAPI` and `mjcPhysics` were available from the standalone
converter plugin directory
- Existing kitless MJCF converter suite: 12 passed
- Existing experience-file tests: 6 passed
- Changelog validation against `upstream/develop`: passed
- Full repository formatting and pre-commit gate: passed

No new regression test was added, as requested.

## Checklist

- [x] I have read and understood the contribution guidelines
- [x] I have run the pre-commit checks
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] I have added a changelog fragment under
`source/isaaclab/changelog.d/`
- [x] My name already exists in `CONTRIBUTORS.md`

(cherry picked from commit 493e3e3)

Author: kellyguo11

apps/isaaclab.python.headless.kit

@@ -198,8 +198,6 @@ enabled=true # Enable this for DLSS
 ########################
 [dependencies]
 "isaacsim.core.version" = {}
-# USD schema plugins must load before the schema registry is first accessed.
-"omni.usd.schema.mujoco" = {}
 
 # Asset path
 # set the S3 directory manually to the latest published S3

apps/isaaclab.python.kit

@@ -16,8 +16,6 @@ keywords = ["experience", "app", "usd"]
 "isaacsim.gui.components" = {}
 "isaacsim.simulation_app" = {}
 "isaacsim.storage.native" = {}
-# USD schema plugins must load before the schema registry is first accessed.
-"omni.usd.schema.mujoco" = {}
 
 # Kit based editor extensions
 "omni.anim.curve.core" = {}

source/isaaclab/changelog.d/fix-duplicate-mujoco-schema-registration.rst

@@ -0,0 +1,5 @@
+Fixed
+^^^^^
+
+* Fixed duplicate ``mjcPhysics`` schema registration during Kit visualizer startup while preserving
+  schema discovery for both installed and source Isaac Sim runtimes.

source/isaaclab/isaaclab/__init__.py

@@ -93,11 +93,29 @@ def _expose_mujoco_usd_schemas():
     OpenUSD reads the search path while building the registry, so this only helps while the
     registry is still unbuilt. A host that queries a schema before importing Isaac Lab has to put
     the plugin directory on ``PXR_PLUGINPATH_NAME`` itself.
+
+    Isaac Lab's Kit experiences must not also enable ``omni.usd.schema.mujoco``. The extension
+    bundles a second plugin with the same name and reports its duplicate registration as an error.
     """
+    plugins = None
     spec = importlib.util.find_spec("mujoco_usd_converter")
-    if spec is None or spec.origin is None:
-        return
-    plugins = os.path.join(os.path.dirname(spec.origin), "plugins")
+    if spec is not None and spec.origin is not None:
+        plugins = os.path.join(os.path.dirname(spec.origin), "plugins")
+
+    # Source Isaac Sim exposes this prebundle only after Kit starts, which is too late for the
+    # schema registry. Its root is available earlier through the launcher environment.
+    if plugins is None or not os.path.isdir(plugins):
+        isaac_path = os.environ.get("ISAAC_PATH")
+        if isaac_path is None:
+            return
+        plugins = os.path.join(
+            isaac_path,
+            "exts",
+            "isaacsim.pip.newton",
+            "pip_prebundle",
+            "mujoco_usd_converter",
+            "plugins",
+        )
     if not os.path.isdir(plugins):
         return
     search_path = os.environ.get("PXR_PLUGINPATH_NAME", "")

tools/wheel_builder/res/__init__.py

@@ -84,7 +84,36 @@ def _should_demote(path: str) -> bool:
         os.environ["PYTHONPATH"] = os.pathsep.join(env_clean + env_demoted)
 
 
+def _expose_mujoco_usd_schemas():
+    """Put the MuJoCo USD schemas on OpenUSD's plugin search path."""
+    plugins = None
+    spec = find_spec("mujoco_usd_converter")
+    if spec is not None and spec.origin is not None:
+        plugins = os.path.join(os.path.dirname(spec.origin), "plugins")
+
+    # Source Isaac Sim exposes this prebundle only after Kit starts, which is too late for the
+    # schema registry. Its root is available earlier through the launcher environment.
+    if plugins is None or not os.path.isdir(plugins):
+        isaac_path = os.environ.get("ISAAC_PATH")
+        if isaac_path is None:
+            return
+        plugins = os.path.join(
+            isaac_path,
+            "exts",
+            "isaacsim.pip.newton",
+            "pip_prebundle",
+            "mujoco_usd_converter",
+            "plugins",
+        )
+    if not os.path.isdir(plugins):
+        return
+    search_path = os.environ.get("PXR_PLUGINPATH_NAME", "")
+    if plugins not in search_path.split(os.pathsep):
+        os.environ["PXR_PLUGINPATH_NAME"] = os.pathsep.join(filter(None, (search_path, plugins)))
+
+
 _deprioritize_prebundle_paths()
+_expose_mujoco_usd_schemas()
 
 
 # TODO(myurasov-nv): bootstrap_kernel() is ported from the internal GitLab wheel builder