Remove dead document_grounded_sdg registration of GenselectAnswersStage

[Anilreddy2309]

Summary

GenselectAnswersStage (nvflow/recipes/finance/stages/sdg/genselect_answers.py) was registered twice:

• recipe=finance, workflow=template_based_sdg, stage=genselect_answers — actively used (template-based-sdg*.yaml)
• recipe=finance, workflow=document_grounded_sdg, stage=genselect_answers — appears unreachable

Every document-grounded-sdg*.yaml workflow uses gym_genselect_answers instead (registered separately in nvflow/generic_stage/sdg/document_grounded/__init__.py), and document-grounded-sdg.yaml's own pipeline comment says it "replaces genselect_answers for DG-SDG". I couldn't find any config, test, or doc referencing finance.document_grounded_sdg.genselect_answers specifically — docs/recipes/finance/stages/template-based-sdg.md documents only the template_based_sdg registration for this stage.

This PR removes the second, apparently-dead @StageRegistry.register(...) decorator.

Flagging for a maintainer to confirm rather than assuming: if this was intentionally left registered for backward compatibility (e.g. an external caller invoking nflow run genselect_answers with an older document-grounded-sdg config), happy to close this instead and add a comment explaining why it's kept. I didn't find evidence of that in the repo, but wanted to call it out explicitly since it's a behavior-affecting removal rather than pure cleanup.

Verification

nflow list-stages --recipe finance --workflow document_grounded_sdg returns the same 7 stages before and after this change (dg_sdg_preprocess, generate_verified_questions, generate_answers, gym_genselect_answers, evaluate_answers, aggregate_answers, dgsdg_post_process) — matching the documented pipeline in docs/development/sdg/document_grounded/ADDING_A_DOMAIN.md.

Test plan

• ruff check / ruff format --check pass
• Manually confirmed finance.template_based_sdg.genselect_answers still resolves via nflow stage-info
• Manually confirmed finance.document_grounded_sdg.genselect_answers now correctly 404s via nflow stage-info (it was never a valid path in any shipped config)
• Full pytest tests/ suite passes (339 passed, 3 pre-existing skips) with no regressions

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