SDG data designer providers addition

[rapaul-nv]

Summary

• Add optional providers: support to sdg/data_designer so configs can register custom Data Designer model providers.
• Allow models[].provider to target either a built-in provider such as nvidia or a custom provider declared in YAML.
• Add a commented OpenAI-compatible endpoint example in customer_support_tools.yaml.
• Add static tests for provider shape, model-provider references, and API key handling.

Notes

providers[].api_key should be an environment variable name such as OPENAI_API_KEY, not ${oc.env:OPENAI_API_KEY}, so Data Designer resolves the secret at request time instead of materializing it in the compiled config.

Validation

• uv run pytest tests/steps/sdg/test_data_designer.py
• uv run ruff check src/nemotron/steps/sdg/data_designer/step.py tests/steps/sdg/test_data_designer.py
• Verified provider wiring locally: providers[].name is registered and models[].provider selects that provider.

[mikemckiernan]

Thank you!