fix(review): add infeasible_steps to copy_result defensive copy

The field was missing from the list-field copy set, allowing cached
results to share the same list object across callers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Trecek

src/autoskillit/recipe/_api_cache.py

@@ -63,6 +63,7 @@ def copy_result(self, result: Any) -> dict[str, Any]:
             "requires_packs",
             "requires_features",
             "deferred_guards",
+            "infeasible_steps",
         ):
             if list_key in r:
                 r[list_key] = list(r[list_key])