feat(eval): support Dynamo backend in run_eval

[jthomson04]

Summary

• add eval_dynamo.yaml for running eval against an external DynamoGraphDeployment
• keep the existing vLLM eval text-generation path intact
• add a small backend setup factory and a generic text-only token-ID generation path for Dynamo and future GenerationInterface.generate() backends
• fix run_eval.py CLI override handling so --config is not folded into OmegaConf overrides
• cover basic backend setup, CLI override parsing, Dynamo multimodal rejection, and token-ID generation/decoding in unit tests

Validation

• NRL_IGNORE_VERSION_MISMATCH=1 uv run pytest tests/unit/evals/test_eval.py -q
• NRL_IGNORE_VERSION_MISMATCH=1 uv run pytest tests/unit/test_config_validation.py -q
• uv run ruff check nemo_rl/evals/eval.py tests/unit/evals/test_eval.py examples/run_eval.py
• real Dynamo smoke: examples/run_eval.py against jothomson-dyn-nano-v2-mx, score=1.0000 (1.0/1)