Fix runtime episode step limit off-by-one

[zjn20030811]

Summary

• count each completed runtime step before evaluating episode termination
• make max_episode_steps=N apply exactly N actions instead of N+1
• preserve environment-driven early completion
• add CPU-only regressions for limits 1, 2, and 3 plus natural termination

The previous loop incremented _episode_steps only after _step returned, while _step checked the old value. That let every bounded episode issue one extra action, which is especially undesirable for real robot rollouts.

Testing

• python -m pytest packages/openpi-client/src/openpi_client/runtime/runtime_test.py -q (4 passed)
• ruff check with the repository configuration
• ruff format --check with the repository configuration