fix: isolate G1 motion tracking backend hooks

[TATP-233]

Summary

• move G1 motion tracking motion body-id resolution out of tracking.py and into backend-owned get_motion_body_ids() hooks
• keep G1 motion tracking env backend-agnostic by removing explicit backend-name checks and leaving action-scale behavior unchanged
• add initialization and cross-backend coverage for motion body-id resolution, including G1 motion tracking reset/step smoke on both backends

Linked Work

• Issue: Closes Work: 消除 G1 motion tracking env 中的 backend-specific 分支泄漏 #217, Closes Infra: Eliminate backend-type check in motion tracking task layer #228
• Milestone:

Validation

• make check
• make test
• Additional task-specific validation listed below

Commands actually run:

make check
make test
uv run pytest tests/envs/test_env_configs.py -q -k g1_motion_tracking
uv run pytest tests/base/test_sim_backend.py -q -m slow -k motion_body_ids_match_motion_xml

Impact

• Backend impact: both
• Platform impact: both
• Training effect expected: no

Artifacts

• W&B:
• benchmark result:
• video / screenshot:
• ONNX / checkpoint:

Checklist

• Added or updated tests where needed
• Updated docs if behavior or workflow changed
• Linked the driving issue
• Noted any follow-up work explicitly