Fix MockSensorContact to match Newton's nested sensing_objs structure

[ooctipus]

Description

the test is using old newton contact sensor api, and new api only accepts nested list structure.

Fixes #4908

Type of change

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

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Greptile Summary

This PR fixes MockNewtonContactSensor so that sensing_objs and counterparts use the nested list[list[tuple[int, int]]] structure that Newton's real SensorContact API returns, matching the double-loop flattening ([obj for world_objs in ... for obj in world_objs]) introduced in PR #4883's ContactSensor._create_buffers. The previous flat list[tuple] caused a TypeError: cannot unpack non-iterable int object when the outer loop tried to iterate over tuples directly.

• Changed self.sensing_objs from [(i, 1) for i in range(n)] to [[(i, 1) for i in range(n)]] — a single-world outer wrapper around the existing flat list
• Same change applied to self.counterparts
• The type annotations are updated correspondingly (list[list[tuple[int, int]]])
• The mock correctly models a single-world scenario, which is sufficient for the unit test use-case

Confidence Score: 5/5

• This PR is safe to merge — it is a minimal, targeted mock fix with no production code changes.
• The change is a single-file, two-line fix to a test mock. It correctly aligns the mock's data structure with the real Newton API (list[list[tuple]]), which is exactly what the _create_buffers double-loop flattening expects. No logic changes are made to production code, and the fix is verified by reading both the mock and the consumer code in contact_sensor.py.
• No files require special attention.

Important Files Changed

Filename	Overview
source/isaaclab_newton/isaaclab_newton/test/mock_interfaces/mock_newton.py	Wraps sensing_objs and counterparts in an outer list to match Newton's list[list[tuple]] structure, fixing TypeError when ContactSensor._create_buffers runs its double-loop flattening.

Sequence Diagram

sequenceDiagram
    participant Test as Unit Test
    participant Mock as MockNewtonContactSensor
    participant CS as ContactSensor._create_buffers()

    Test->>Mock: MockNewtonContactSensor(num_sensing_objs, num_counterparts)
    Note over Mock: sensing_objs = [[(i,1) for i in range(n)]]<br/>counterparts = [[(i,1) for i in range(m)]]

    Test->>CS: _create_buffers()
    CS->>Mock: contact_view.sensing_objs
    Mock-->>CS: list[list[tuple[int,int]]]
    CS->>CS: flat_sensing = [obj for world_objs in sensing_objs for obj in world_objs]
    Note over CS: Double-loop flattening works correctly

    CS->>Mock: contact_view.counterparts
    Mock-->>CS: list[list[tuple[int,int]]]
    CS->>CS: flat_counterparts = [obj for world_objs in counterparts for obj in world_objs]
    Note over CS: Double-loop flattening works correctly

    CS-->>Test: Buffers initialized successfully

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_{Last reviewed commit: 94e5969}