feat: Replace BDX_R robot model and fix viewer scripts

[KaydenKnapik]

Description

This pull request replaces the entire old BDX robot model (URDF and assets) with the new, more detailed version.

This process also involved debugging and fixing several issues to make the new model compatible with the existing gait generator and viewer scripts:

• Fixed URDF Structure: Resolved a "multiple root links" error by correcting link names (right_foot, left_foot).
• Standardized Naming: Renamed the root link to body_r and all movable joints to snake_case (e.g., left_hip_yaw) to match the exact names expected by placo_walk_engine.py.
• Fixed Replay Script: Corrected an AttributeError in replay_motion.py by changing the method call pushFrame to push_frame.
• System Dependencies: The replay viewer requires the freeglut3-dev system package to be installed on Ubuntu.

After these changes, the gait generator and both replay_motion.py and ref_motion_viewer.py work correctly with the new model.

Fixes: N/A

Checklist

Please check off each item ([x]) once complete, or mark it as [N/A] if it doesn't apply:

• Added your name to CONTRIBUTORS.md (alphabetically by first name)
• Updated CHANGELOG.md:
  • Global changelog (if your change affects the overall repo)
  • Model-specific changelog (if it affects a specific model only)
• Followed the XML formatting/style guidelines
• Ran uv run test/model_test.py locally and ensured all tests pass - Note: Manually tested by successfully running the gait generator and motion viewers, which now work as expected.

Refer to the contributing guide if you're unsure about any of the steps.