Fail recipe if no output produced, add tests

- execute_recipe now returns success=False if output_cid is None
- Add TestRecipeOutputRequired tests to catch missing output
- Recipe must produce valid output to be considered successful

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

legacy_tasks.py

@@ -905,6 +905,21 @@ def execute_recipe(self, recipe_sexp: str, input_hashes: Dict[str, str], run_id:
         output_ipfs_cid = output_result.get("ipfs_cid")
         output_path = output_result.get("path")
 
+    # Fail if no output was produced
+    if not output_cid:
+        logger.error(f"Recipe produced no output! output_step={plan.output_step_id}, result={output_result if output_step else 'no output step'}")
+        return {
+            "success": False,
+            "run_id": run_id,
+            "error": "Recipe produced no output",
+            "plan_cid": plan_cid,
+            "plan_sexp": plan_sexp,
+            "step_results": step_results,
+            "total_steps": len(plan.steps),
+            "cached": total_cached,
+            "executed": total_executed,
+        }
+
     # ============ Phase 6: Store Results ============
     logger.info("Phase 5: Storing results...")