Fix safe environment exports and Gymnasium API

[OtisMcLeary-123]

Summary

• restore exports for the five safety-aware Panda environments
• replace stale gym/gym_robotics core usage with the current Gymnasium core
• fix the accidental StackSafeStack3 registration caused by a missing comma
• preserve the safety cost in the Gymnasium info mapping

Verification

• 6 focused registry/reset/step tests pass
• all 20 safe variants (5 tasks x sparse/dense x ee/joints) reset and execute one headless step on Python 3.12, Gymnasium 1.3.0, and PyBullet 3.2.7

The patch intentionally leaves the legacy modules in place but routes registered safe environments through a Gymnasium-compatible constructor module.