Add ExactMatch Metric scorer PoC

[SandyChapman]

Summary

• add an undecorated ExactMatchMetric implementing the shared MetricInput -> MetricResult contract
• add an ExactMatchScorer BYOB wrapper via the class-based scorer adapter
• add an example benchmark covering preannotated scorer, inline class scorer, and configured metric instance modes

Validation

• .venv/bin/ty check src/nemo_evaluator/metrics/exact_match.py src/nemo_evaluator/scorers.py src/nemo_evaluator/init.py src/nemo_evaluator/metrics/init.py tests/test_scoring/test_metric_contract.py tests/test_environments/test_custom_metric_contract.py tests/test_integration/test_eval_loop_integration.py
• .venv/bin/pyright --pythonpath .venv/bin/python src/nemo_evaluator/metrics/exact_match.py src/nemo_evaluator/scorers.py src/nemo_evaluator/init.py src/nemo_evaluator/metrics/init.py tests/test_scoring/test_metric_contract.py tests/test_environments/test_custom_metric_contract.py tests/test_integration/test_eval_loop_integration.py
• .venv/bin/ruff check CHANGELOG.md src/nemo_evaluator/metrics/exact_match.py src/nemo_evaluator/scorers.py src/nemo_evaluator/init.py src/nemo_evaluator/metrics/init.py tests/test_scoring/test_metric_contract.py tests/test_environments/test_custom_metric_contract.py tests/test_integration/test_eval_loop_integration.py examples/benchmarks/exact_match_metric_poc.py
• .venv/bin/python -m pytest tests/test_scoring/test_metric_contract.py tests/test_environments/test_custom_metric_contract.py tests/test_integration/test_eval_loop_integration.py -v -m "not network"
• executed examples.benchmarks.exact_match_metric_poc through run_evaluation; all three PoC modes returned rewards [1.0, 1.0] and pass@1 1.0

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[SandyChapman]

This is pretty much the same as #2 above, but shows annotating an instance rather than the type.