Fix SaveLinkSpec registration crash on ResultContainer string indexing

AGENTS.md

@@ -91,6 +91,7 @@ Handles dispatch → process → collect workflow for B-Fabric applications. Use
 - Line length: 120 (ruff and black)
 - basedpyright uses per-package baseline files at `.basedpyright/baseline.{package}.json`
 - Integration tests live in a separate repository
+- Use TDD: write a failing test first, verify it fails, then fix the code, then verify the test passes
 
 ## Branches
 

bfabric_app_runner/src/bfabric_app_runner/output_registration/register.py

@@ -154,7 +154,7 @@ def _save_link(spec: SaveLinkSpec, client: Bfabric, workunit_definition: Workuni
     if existing_link_id is not None:
         link_data["id"] = existing_link_id
     res = client.save("link", link_data)
-    logger.info(f"Link {spec.name} saved with id {res['id']} for entity {entity_type} with id {entity_id}")
+    logger.info(f"Link {spec.name} saved with id {res[0]['id']} for entity {entity_type} with id {entity_id}")
 
 
 def find_default_resource_id(workunit_definition: WorkunitDefinition, client: Bfabric) -> int | None:

tests/bfabric_app_runner/output_registration/test_save_link.py

@@ -0,0 +1,39 @@
+from unittest.mock import MagicMock
+
+import pytest
+
+from bfabric.results.result_container import ResultContainer
+from bfabric_app_runner.output_registration.register import _save_link
+from bfabric_app_runner.specs.outputs_spec import SaveLinkSpec
+
+
+@pytest.fixture()
+def mock_client():
+    return MagicMock()
+
+
+@pytest.fixture()
+def mock_workunit_definition():
+    mock = MagicMock()
+    mock.registration.workunit_id = 42
+    return mock
+
+
+@pytest.fixture()
+def spec():
+    return SaveLinkSpec(name="test_link", url="https://example.com", entity_type="Workunit")
+
+
+def test_save_link_no_existing(mock_client, mock_workunit_definition, spec):
+    """Test that _save_link works when no existing link is found."""
+    mock_client.read.return_value = ResultContainer(results=[], total_pages_api=None, errors=[])
+    mock_client.save.return_value = ResultContainer(results=[{"id": 123}], total_pages_api=None, errors=[])
+
+    _save_link(spec, mock_client, mock_workunit_definition)
+
+    mock_client.save.assert_called_once()
+    saved_data = mock_client.save.call_args[0][1]
+    assert saved_data["name"] == "test_link"
+    assert saved_data["url"] == "https://example.com"
+    assert saved_data["parentid"] == 42
+    assert saved_data["parentclassname"] == "Workunit"