fix(runners): widen Runner.__init__ agent param to accept BaseNode

The `Runner.agent` class field is typed `Optional[BaseAgent | 'BaseNode']`
and the runtime explicitly branches on `isinstance(self.agent, BaseNode)`
in both `run_async` and `run_live`, so a `BaseNode` root (e.g. a
`Workflow`) works at runtime. However, the `__init__` `agent` parameter
was still typed `Optional[BaseAgent]`, so type checkers reject
`Runner(agent=my_workflow)`.

Widen the `__init__` `agent` annotation to `Optional[BaseAgent | 'BaseNode']`
to match the class field, mirroring the existing forward-reference style
used for the field and the `run_live` `node` parameter. This is a
type-only change with no runtime behavior change.

Fixes #6270

Author: anxkhn

src/google/adk/runners.py

@@ -239,7 +239,7 @@ def __init__(
       *,
       app: Optional[App] = None,
       app_name: Optional[str] = None,
-      agent: Optional[BaseAgent] = None,
+      agent: Optional[BaseAgent | BaseNode] = None,
       node: BaseNode | None = None,
       plugins: Optional[List[BasePlugin]] = None,
       artifact_service: Optional[BaseArtifactService] = None,
@@ -320,7 +320,7 @@ def _require_root_agent(self) -> BaseAgent:
   def _resolve_app(
       app: Optional[App],
       app_name: Optional[str],
-      agent: Optional[BaseAgent],
+      agent: Optional[BaseAgent | BaseNode],
       node: BaseNode | None,
       plugins: Optional[List[BasePlugin]],
   ) -> App:

tests/unittests/test_runners.py

@@ -1777,6 +1777,22 @@ def test_resolve_app_with_agent_wraps_in_app(self):
     assert runner.app_name == "test_app"
     assert runner.agent is self.root_agent
 
+  def test_resolve_app_with_base_node_via_agent_param(self):
+    """Test that a BaseNode root passed via `agent` is accepted."""
+    from google.adk.workflow._base_node import BaseNode
+
+    node = BaseNode(name="test_node")
+    runner = Runner(
+        app_name="test_app",
+        agent=node,
+        session_service=self.session_service,
+        artifact_service=self.artifact_service,
+    )
+    assert runner.app is not None
+    assert runner.app.root_agent is node
+    assert runner.app_name == "test_app"
+    assert runner.agent is node
+
   def test_resolve_app_with_node_wraps_in_app(self):
     """Test that a bare node is wrapped into an App."""
     from google.adk.workflow._base_node import BaseNode