Tune Velociraptor MJCF leg joint defaults and lock with regression coverage

[Copilot]

The request was to review raptor.xml and apply practical model improvements. This PR makes a small, stability-focused MJCF tuning change and adds a targeted regression check so the tuned values remain intentional.

• MJCF tuning (raptor leg joints)

  • Updated environments/velociraptor/assets/raptor.xml leg joint defaults:
    • damping: 5.0 → 6.0
    • stiffness: 15.0 → 18.0
  • Scope is intentionally narrow: only the shared leg_joint default was adjusted.

• Regression guard for model parameters

  • Added a focused environment test that inspects MuJoCo-loaded joint data for r_hip_pitch and asserts the tuned values are present.
  • Prevents silent drift in core leg stability parameters during future XML edits.

• Illustrative diff

  <default class="leg_joint">

```

Original prompt

Please review the raptor xml and see if you have any improvements

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