Add state handling to public VLA API

[LiangSu8899]

Summary

• attach state from VLAModel.predict() to observation dicts without overwriting explicit observation state
• refresh prompt prefixes when set_prompt(..., state=...) frontends receive a changed state value
• document the state contract for Pi0, Pi0.5, Pi0-FAST, GROOT N1.6, and GROOT N1.7

Validation

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python3 -m pytest tests/test_load_model_use_fp8_kwarg.py -q (18 passed)
• StableHLO: Pi0 RTX vs official PyTorch FP32 ref cosine 0.997490
• StableHLO: Pi0.5 RTX vs official PyTorch FP32 ref cosine 0.999705
• StableHLO: GROOT N1.6 RTX DiT/action path vs rtx_groot_ref.npz cosine 0.999248
• StableHLO: Pi0-FAST JAX vs JAX bf16 ref worst segment cosine 0.993657
• git diff --check