Add test coverage for WorkflowRunner

[Anilreddy2309]

Summary

WorkflowRunner (nvflow/core/workflow_runner.py, ~500 lines) is the most complex piece of core orchestration logic in the repo — config loading with _base_ inheritance, dynamic stage expansion from models/checkpoints sections, dependency-closure validation, and stage execution — and had no dedicated test file. It was only exercised incidentally by tests written for unrelated features (tests/test_grounding_feature_gate.py, nvflow/recipes/multimodal/tests/test_hopchain_image_filter_paths.py), none of which touch run(), _expand_dynamic_stages(), _load_config_with_inheritance(), or the dependency-closure walk in _validate_stages().

What's covered

tests/test_workflow_runner.py (44 tests):

• Config loading & _base_ inheritance — simple load, missing file, single-level and chained _base_ merging (child overrides base, non-overridden fields survive), missing base file, relative path resolution, OmegaConf interpolation resolution
• Dynamic stage expansion — models section expansion, workflow-level defaults forwarded but overridden by entry config, structural keys (recipe/cluster/etc.) not leaking into expanded stages, explicit stages: config not clobbered by expansion, checkpoints eval_steps (single int and list) expanding both stages and pipeline_stages
• _get_expname, _resolve_env_names, _get_run_after_names — including per-environment dependency name expansion
• _validate_stages / validate_config — valid stages pass, stage missing from pipeline config, unregistered stage, missing stage config block, transitive dependency-closure walk, dependency missing config block, dependency not registered, all 5 required top-level fields
• run() — full pipeline execution order, subset runs, dependency filtering to only stages submitted this session, unknown-environment validation, the unregistered-sibling-stage preflight warning, fail-fast on an invalid requested stage before any stage executes

Design notes

Two construction strategies keep the suite fast and each fixture minimal: config-loading tests instantiate WorkflowRunner normally against real YAML files under tmp_path (since that I/O is what's being tested); everything downstream uses a _runner_from_config() helper that builds a WorkflowRunner from an in-memory dict via __new__ + the same post-load steps __init__ performs, skipping file I/O.

StageRegistry is a process-wide class-level registry, so tests that register stages use an isolated_registry fixture (monkeypatches StageRegistry._stages to a fresh dict for the test's duration) so registrations never leak into other tests or the real registry.

Test plan

• ruff check / ruff format --check pass
• mypy passes cleanly (one documented type: ignore[return-value] on a small typed accessor helper, explained inline — WorkflowRunner.config's declared type is a loose OmegaConf.to_container() union that mypy can't chain-index outside the constructor's own local flow; this is upstream typing looseness, not something this PR should try to fix by touching nvflow/core/workflow_runner.py's public API)
• License header check passes
• New suite passes (44/44) in an environment mirroring the exact CI install (pip install -e . --no-deps + tests/requirements-ci.txt, no nemo-skills/torch)
• Full pytest tests/ suite passes (383 passed, 3 pre-existing skips) — no regressions

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