Builds an Nx.Serving for classifier inference.

Important options:

• :model_version - Model architecture version. Defaults to 1 for backward compatibility.
• :decision_policy - Optional persisted policy from a trained classifier. v7 uses this for calibrated positive thresholds and low-signal safeguards.
• :positive_threshold - Override the positive class threshold.
• :min_positive_tokens - Override the minimum active-token count needed for a positive v7 prediction.
• :vector_length - Fixed input length. Defaults to 512.