Fail on unmerged LoRA keys during JAX conversion

[J0YY]

Summary

• Capture load_state_dict results in the JAX to PyTorch converter
• Raise a clear error when unexpected LoRA keys remain after conversion
• Prevent saving PyTorch checkpoints that would silently drop LoRA adapters

Scope

• This is a guard against incorrect converted weights
• It does not implement LoRA adapter merging

Verification

• python3 -m py_compile examples/convert_jax_model_to_pytorch.py
• git diff --check

Addresses #958