fix(workflows): StepRegistry.add tolerates a corrupted non-dict existing entry (#3630)

StepRegistry.add read existing = self.data['steps'].get(step_id, {}) then called
existing.get('installed_at', ...). A corrupted-but-parseable registry holding a
non-dict entry (e.g. {'steps': {'foo': 'corrupted'}}) — which _load() accepts,
since it validates only the top-level dict and that 'steps' is a dict — made
add() raise AttributeError. WorkflowRegistry.add was hardened for exactly this
(#3419); mirror its isinstance guard so a non-dict existing entry is treated as
absent.

Test copies the WorkflowRegistry sibling test for StepRegistry (fails before:
AttributeError on existing.get()).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/specify_cli/workflows/catalog.py

@@ -894,7 +894,11 @@ class StepRegistry:
         import copy
         from datetime import datetime, timezone
 
-        existing = self.data["steps"].get(step_id, {})
+        raw_existing = self.data["steps"].get(step_id)
+        # Corrupted-but-parseable registries may hold non-dict entries; treat
+        # them as absent rather than crashing on existing.get() (mirrors
+        # WorkflowRegistry.add).
+        existing = raw_existing if isinstance(raw_existing, dict) else {}
         metadata_to_store = copy.deepcopy(metadata)
         metadata_to_store["installed_at"] = existing.get(
             "installed_at", datetime.now(timezone.utc).isoformat()