feat(evaluator): resolve model refs for plugin submit targets

[SandyChapman]

Summary

• allow evaluator plugin submit targets to accept platform ModelRef values
• resolve ModelRef targets to concrete SDK Model values before building the evaluator job spec
• keep SDK/job execution target types concrete and add coverage for config validation

Testing

• uv run --frozen pytest plugins/nemo-evaluator/tests/test_sdk.py plugins/nemo-evaluator/tests/test_evaluate_job.py -q
• uv run --frozen ruff check plugins/nemo-evaluator/src/nemo_evaluator/sdk/_executor.py plugins/nemo-evaluator/src/nemo_evaluator/sdk/resources.py plugins/nemo-evaluator/tests/test_sdk.py
• uv run --frozen --extra cpu ty check plugins/nemo-evaluator/src/nemo_evaluator/sdk/_executor.py plugins/nemo-evaluator/src/nemo_evaluator/sdk/resources.py plugins/nemo-evaluator/tests/test_sdk.py

[github-actions]

Suite	Lines Covered	Line Rate	Branch Rate
Unit Tests	18419/24402	75.5%	62.0%
Integration Tests	11853/23179	51.1%	26.3%

[SandyChapman]

Closing as superseded by #128. The ModelRef submit-target support from this PR has been folded into the broader evaluator ref-contract cleanup there.