Adds parsing of instanced meshes to fetching prims utils

[Mayankm96]

Description

This MR does the following:

• Adds parsing of instanced prims in isaaclab.sim.utils.get_all_matching_child_prims and isaaclab.sim.utils.get_first_matching_child_prim. Earlier, instanced prims were skipped since Usd.Prim.GetChildren does not return instanced prims.
• Adds parsing of instanced prims in isaaclab.sim.utils.make_uninstanceable to make all prims uninstanceable.

These are needed to parse meshes for the dynamic raycaster class in #3298

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[Copilot]

Pull Request Overview

This PR enhances prim traversal utilities to include support for instanced prims, which were previously skipped during traversal operations.

• Replaces GetChildren() with GetFilteredChildren(Usd.TraverseInstanceProxies()) to include instanced prims
• Updates three utility functions: get_all_matching_child_prims, get_first_matching_child_prim, and make_uninstanceable
• Adds comprehensive test coverage for the new instanced prim functionality

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
source/isaaclab/isaaclab/sim/utils.py	Updates three functions to traverse instanced prims using GetFilteredChildren()
source/isaaclab/test/sim/test_utils.py	Adds test cases for instanced prim traversal and imports UsdPhysics module
source/isaaclab/docs/CHANGELOG.rst	Documents the changes for version 0.46.0
source/isaaclab/config/extension.toml	Bumps version number to 0.46.0
source/isaaclab/utils/assets.py	Removes copyright header (appears to be incomplete file content)

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[renezurbruegg]

LGTM! I have this changes locally since a few months and never had any issues.

Only thing to discuss, I think it could be helpful to add an argument "skip_instanced_prims" or similar, to recover the old functionality of the methods if anyone needs it.