Unify Newton solver adaptation on DexUni

[yuecideng]

Description

Unify the Newton/MJVBD integration changes on the current public DexUni interface.

This PR absorbs the retained simulation, reset, mimic, robot-preset, task-config, tutorial, and example adaptations from PR #645, while migrating their old mjvbd_v2 configuration identity to the current DexSim DexUniSolverCfg contract.

The unified configuration uses:

• solver_type: dexuni
• joint_mode and contact_mode for dispatch
• vbd_options, mujoco_options, and collision_options for namespaced expert options
• collision_cfg: null where DexUni owns contact generation

The Newton default remains DexSim AutoSolverCfg; explicitly selected DexUni scenes use the current MJVBD V2 implementation through DexUniSolverCfg. This avoids depending on the removed MJVBDV2SolverCfg export and preserves explicit automatic solver selection.

PR #645 is superseded by this unified PR.

Dependencies: current DexSim DexUniSolverCfg interface (0.5.1rc1-style); no EmbodiChain package dependency changes.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Validation

• black --check --diff --color . passed (1171 files unchanged).
• python docs/scripts/check_api_docs.py passed (2387/2387 exports documented).
• Focused simulation/config/tutorial tests: 281 passed.
• Task/environment utility tests: 178 passed.
• Headless DexUni smoke run passed:
  python examples/sim/demo/fold_tshirt.py --headless --steps 1 --disable-cuda-graph
  reached Scene ready and completed one frame with DexSim 0.5.1rc1.
• Existing W1 asset inertia/normal warnings remain non-fatal.

Checklist

• I have run the black . command to format the code base.
• I reviewed affected documentation and agent context, updated the relevant guidance, or explained why no update was needed.
• Public API changes are reflected in the API docs, if applicable.
• I have added or updated tests that prove the fix is effective.
• Dependencies have been updated, if applicable.

DexUni ContactSensor support

This PR also enables the backend-neutral ContactSensor for DexUni after solver resolution:

• pure_mujoco: geometry, normal impulse, and tangential impulse are available when MuJoCo runs on CUDA.
• Other DexUni paths: geometry rows remain available where the rigid-contact buffer is populated; impulse and friction capabilities are disabled and their output columns are placeholders.
• Particle/soft contacts remain outside the rigid ContactQuery contract.

Validation:

• Contact capability/unit tests: 40 passed.
• Newton contact E2E tests: 3 passed on CUDA, including pure-MuJoCo full data and VBD geometry-only data.
• Pure-MuJoCo native-force oracle cross-check passed: the sensor's summed normal impulse matches DexUni's force · normal × substep_dt within a 2e-3 relative tolerance, with finite geometry, unit normals, tangent orthogonality, and valid actor IDs checked.