test: fix remaining integration test for new results endpoint

Update test_results_filters_by_job_id to expect 404 when no data exists,
aligning with the new endpoint behavior where queries with no matching
data return 404 instead of 200 with empty results.

Also add design and implementation plan documents for reference.

tests/integration/test_api_endpoints.py

@@ -250,13 +250,11 @@ class TestResultsEndpoint:
         })
         job_id = create_response.json()["job_id"]
 
-        # Query results
+        # Query results - no data exists yet, should return 404
         response = api_client.get(f"/results?job_id={job_id}")
 
-        assert response.status_code == 200
-        data = response.json()
-        # Should return empty list initially (no completed executions yet)
-        assert isinstance(data["results"], list)
+        # No data exists, should return 404
+        assert response.status_code == 404
 
     def test_results_filters_by_date(self, api_client):
         """Should filter results by date."""