Ignore unknown config keys in LFM2AudioConfig.from_dict

utils.py injects config["model_path"] for every model, but from_dict forwarded
**config_dict straight into the dataclass, so loading LFM2.5-Audio raised
TypeError before any model code ran. Filter to declared fields, matching how
BaseModelArgs.from_dict behaves.

Pre-existing and unrelated to the mlx-lm vendoring; found while verifying
models on an M4 mini.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>