fix(core): recover from hallucinated tool names instead of ending the scan

A tool call for a name Strix does not register raised ModelBehaviorError
from the SDK turn resolver, which nothing retries: the root agent's raise
tore down the whole scan and a sub-agent died before its status was set.
Opt into the SDK's tool_not_found_behavior="return_error_to_model" so the
unknown call comes back as a tool result and the agent self-corrects.

The setting landed in openai-agents 0.19.0, which requires openai>=2.45,
so both pins move.

tests/test_runner_root_prompt.py

@@ -11,6 +11,7 @@ from typing import Any
 
 import httpx
 import pytest
+from agents import ModelSettings
 from openai import RateLimitError
 
 import strix.tools.notes.tools as notes_tools
@@ -74,7 +75,7 @@ def _patch_engine_scaffold(
 
     monkeypatch.setattr(runner, "build_root_task", lambda _scan_config: "task")
     monkeypatch.setattr(runner, "build_scope_context", lambda _scan_config: scope_context)
-    monkeypatch.setattr(runner, "make_model_settings", lambda *_args, **_kwargs: object())
+    monkeypatch.setattr(runner, "make_model_settings", lambda *_args, **_kwargs: ModelSettings())
 
     captured: dict[str, Any] = {}
 
@@ -87,7 +88,8 @@ def _patch_engine_scaffold(
     monkeypatch.setattr(runner, "make_child_factory", lambda **_kwargs: lambda **_k: object())
     monkeypatch.setattr(runner, "open_agent_session", lambda _root_id, _db: object())
 
-    async def _raise_rate_limit(*_args: Any, **_kwargs: Any) -> None:
+    async def _raise_rate_limit(*_args: Any, **kwargs: Any) -> None:
+        captured["run_config"] = kwargs.get("run_config")
         raise _make_rate_limit_error()
 
     monkeypatch.setattr(runner, "run_agent_loop", _raise_rate_limit)
@@ -176,3 +178,21 @@ async def test_root_prompt_options_default_to_none(
     kwargs = captured["kwargs"]
     assert kwargs["instructions_override"] is None
     assert kwargs["system_prompt_context"] == {"scope": "built-in"}
+
+
+@pytest.mark.asyncio
+async def test_unknown_tool_calls_are_returned_to_the_model(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: Any,
+) -> None:
+    """A hallucinated tool name must not end the scan."""
+    captured = _patch_engine_scaffold(monkeypatch, tmp_path, {})
+
+    await runner.run_strix_scan(
+        scan_config={"targets": [], "scan_mode": "deep"},
+        scan_id="scan-unknown-tool",
+        image="img",
+        coordinator=AgentCoordinator(),
+    )
+
+    assert captured["run_config"].tool_not_found_behavior == "return_error_to_model"