Joint position limits cannot be set by users after spawn

[yuecideng]

Summary

Users are currently unable to set joint position limits after an articulation or robot has already been spawned.

Current behavior

After spawn, the joint position limit cannot be updated through the user-facing API.

Expected behavior

Users should be able to set or update joint position limits after spawn through a supported public API.

Requested change

• Support setting joint position limits after spawn in the next DexSim release.
• Fix the related API issues/bugs for this feature so the behavior is reliable and consistent.

Notes

This is currently a limitation for users who need to adjust joint constraints dynamically after initialization.