feat(scoring): add NEL-NMP integration contracts (Approach 3)

[wprazuch]

Defines the abstract interface between NEL and downstream metric providers (e.g., NMP's nemo_evaluator_sdk) in a new module: src/nemo_evaluator/scoring/contracts.py (~250 LOC).

Provides:

• Metric protocols (runtime_checkable, structural):
  • Metric -- core object-style metric contract (type, metric, compute_scores, score_names)
  • CorpusMetric -- corpus-level aggregation
  • MetricWithSecrets -- secret resolution protocol
  • MetricWithPreflight -- one-time preflight setup
• Pydantic result types (no heavy deps -- no pyarrow, no pandas):
  • MetricScore, MetricResult
  • ScoreStats, RubricScoreValue, RubricScoreStat
• SecretRefLike Protocol (abstract secret reference)
• SecretResolver type alias
• metric_as_scorer() helper: bridges object-style Metric to NEL's function-style (ScorerInput) -> dict scorer protocol

Design rationale (Approach 3):

• NMP's concrete metric implementations (BLEU, ROUGE, F1, RAGAS, LLMJudge, etc.) stay in nemo_evaluator_sdk with their external deps (sacrebleu, rouge_score, openai, ragas).
• NEL owns only the contract surface. NMP SDK will be updated in a follow-up PR to depend on NEL for these types, dropping its own copies and becoming thinner.
• Dependency direction is strictly one-way: SDK -> NEL. NEL never imports from any provider.
• Contracts are dep-light: pure Pydantic models + typing.Protocol, no pyarrow/pandas/openai/etc. leaking into NEL.

Verified:

• 14 new unit tests pass
• SDK's ExactMatchMetric structurally satisfies the Metric Protocol with zero code changes (isinstance check passes)
• All 162 SDK + contracts tests pass together

This commit is stacked on wprazuch/sdk-onboarding-approach1 so the contracts land alongside the verbatim SDK copy for easier review. The follow-up plan is:

1. Merge Approach 1 (verbatim copy) to dev/0.3.0
2. Merge this PR (contracts) to dev/0.3.0
3. Open a matching PR on NMP's nemo_evaluator_sdk to depend on NEL's contracts and drop its redundant abstraction copies

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