Fix Jaco gripper force/torque sensor readings

[Battleplus]

Summary

• move the Jaco gripper force/torque site from the fixed palm child to the right_gripper mounting body
• preserve the site's original world-frame orientation with an explicit quaternion
• add a MuJoCo regression test for the sensor attachment and bounded finite readings

Why this placement

Attaching the sensors to grip_site makes the default readings zero, but that site belongs to a massless visualization-only eef body and does not measure the complete finger subtree. Moving the existing ft_frame one level up measures the complete gripper at its mounting interface while preserving its pose.

With MuJoCo 3.8.0 after mj_resetData + mj_forward:

Sensor	Before	After
force_ee max abs	3.47e17	0.25
torque_ee max abs	3.42e15	0.141

Testing

• python -m pytest tests/test_grippers/test_jaco_sensors.py -q (1 passed)
• black --check tests/test_grippers/test_jaco_sensors.py
• git diff --check

Fixes #808