Fix contact sensor visualization

[matafela]

Description

Fix crash when visualize contact sensor. Example: examples/sim/sensors/create_contact_sensor.py.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Dependencies have been updated, if applicable.

[Copilot]

Pull request overview

This PR fixes a crash that occurred when calling set_contact_point_visibility() on a ContactSensor. The _data_buffer is pre-allocated with max_contact_num entries (initialized with torch.empty), but previously the method read the entire buffer rather than only the _curr_contact_num valid entries. The invalid (uninitialized) env_ids values at indices beyond _curr_contact_num would cause an out-of-bounds index into self._sim.arena_offsets, crashing the program.

Changes:

• Slice _data_buffer["position"] with [: self._curr_contact_num] to read only valid contact positions.
• Slice _data_buffer["env_ids"] with [: self._curr_contact_num] before using the values to index arena_offsets.

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[Copilot]

The PR checklist states "I have added tests that prove my fix is effective," but no test for set_contact_point_visibility was added in the test suite (e.g., tests/sim/sensors/test_contact.py). The existing test_fetch_contact only exercises get_data() and filter_by_user_ids(). A test calling contact_sensor.set_contact_point_visibility(visible=True) after update() should be added to verify the crash is fixed and does not regress.