fix: remove backend hot-path type shims

[TATP-233]

Summary

• remove backend hot-path np.asarray(...) wrappers that were only serving static typing
• keep backend return contracts while avoiding extra runtime calls in MuJoCo and Motrix getters
• use local # type: ignore[no-any-return] only at third-party stub boundaries

Validation

• pre-commit hooks on commit (ruff, ruff format, pyright, mypy)
• UV_CACHE_DIR=.uv-cache uv run --no-sync ruff format --check .
• UV_CACHE_DIR=.uv-cache uv run --no-sync ruff check .
• UV_CACHE_DIR=.uv-cache uv run --no-sync mypy src/unilab

Notes

• stacked on top of fix: remove unnecessary np.asarray in motrix backend & bump motrixsim #293 / fix/motrix-remove-unnecessary-np-asarray

[TATP-233]

Already included via #293 and merged to main.