refactor: extract shared training entrypoint helpers

[TATP-233]

Summary

• extract shared training entrypoint helpers into src/unilab/training/common.py
• add src/unilab/training/backend_adapter.py to centralize motrix_legacy / motrix_play_only config adaptation
• refactor train_rsl_rl.py, train_appo.py, train_offpolicy.py, and train_mlx_ppo.py to use the shared helpers while preserving script-level compatibility wrappers used by tests
• add direct tests for the new training helper modules

Linked Work

• Issue: Work: 提取训练脚本公共逻辑 — 消除 4 脚本重复代码 #150
• Milestone: N/A

Validation

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

Commands actually run:

uv run python -m py_compile scripts/train_rsl_rl.py scripts/train_appo.py scripts/train_offpolicy.py scripts/train_mlx_ppo.py src/unilab/training/common.py src/unilab/training/backend_adapter.py src/unilab/training/__init__.py
uv run ruff check scripts/train_rsl_rl.py scripts/train_appo.py scripts/train_offpolicy.py scripts/train_mlx_ppo.py src/unilab/training/common.py src/unilab/training/backend_adapter.py src/unilab/training/__init__.py
uv run pytest tests/training/test_training_helpers.py tests/scripts/test_train_scripts.py -q
make check
make test
UV_CACHE_DIR=.uv-cache uv run pre-commit run --all-files

Notes:

• UV_CACHE_DIR=.uv-cache uv run pre-commit run --all-files still reports existing pyright / mypy failures outside this change set, mainly under benchmark/, tests, and legacy script typing paths.
• make check and make test pass for the current branch.

Impact

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

Artifacts

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

Checklist

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

Follow-up work:

• clean up the existing pyright / mypy failures that currently cause pre-commit --all-files to fail outside this refactor scope