Expose classifier contract results

[henry3260]

Summary

• add contract-based classify_result() output for EnsembleClassifier using MultiClassResult / ClassificationResult
• add classify_result() to TemplateClassifier while preserving the existing classify() dict API
• add focused tests covering ensemble contract results, OOD mapping, and template classifier compatibility

Compatibility

• Existing predict_with_confidence() tuple callers are unchanged
• Existing TemplateClassifier.classify() dict callers are unchanged

Tests

• .venv/bin/python -m pytest -vv tests/classifier/test_classifier_contract.py
• .venv/bin/python -m pytest tests

Note: ruff is not installed in this local environment; command -v ruff and .venv/bin/ruff were unavailable.