Filter zero-variance groups after infra checks

[nightlessbaron]

Summary

• reject all-correct and all-incorrect rollout groups after infra/abort validation
• preserve the existing abort and infra-failure rejection precedence
• cover every configured infra status plus nested, mixed, and zero-variance groups

Dependency

This is stacked on #60 and therefore temporarily includes #60's commits in the prod diff. Rebase onto prod after #60 lands.

Validation

• pytest -q --confcutdir=tests/fast/rollout tests/fast/rollout/test_dynamic_sampling_filters.py (20 passed)
• Black and Ruff on both changed files
• py_compile
• git diff --check