Load plugins in compile sample

[LouRohanNV]

Summary

This updates sample/compile to load plugin libraries from nearby mujoco_plugin directories before calling mj_parse.

The sample already routes non-MJB inputs through mj_parse and outputs through mj_encode, so this keeps the existing generic decoder path and only makes locally built/installed decoder plugins discoverable.

Fixes #3389

Details

• Checks <compile executable dir>/mujoco_plugin.
• Checks <current working directory>/mujoco_plugin.
• Only calls mj_loadAllPluginLibraries when the directory exists.

USD / non-USD impact

This does not require USD support. In non-USD workflows, if no plugin directory exists, the helper does nothing and XML/URDF/MJB behavior stays on the existing path.

Validation

Built locally:

cmake --build ./mujoco/build --target compile --parallel 8