test: final coverage batch - utils, pipeline, agent, star tests

- 22 unit test files added/modified across all core modules
- Fix jsonschema ValidationError import in test_tool
- Coverage increased from 23% to ~35% overall

tests/test_openai_source.py

@@ -284,7 +284,7 @@ async def test_openai_payload_handles_none_think_content():
                 {
                     "role": "assistant",
                     "content": [
-                        {"type": "think", "think": None},  # type: ignore
+                        {"type": "think", "think": None},
                         {"type": "text", "text": "final answer"},
                     ],
                 }