test(plugins): assert error propagation in on_*_error fallback tests

The three on_*_error "fallback to runner" tests placed their only assertion
inside an `except` block with no guard, so they passed even when no error was
raised, the exact regression they exist to catch. The model-callback variant
only constructed InMemoryRunner (which never runs the agent), so it could
never raise and was fully vacuous.

Rewrite them to fail on a no-raise regression, matching existing repo
conventions:
- test_async/live_on_tool_error_fallback_to_runner: wrap the call in
  `pytest.raises(ClientError)` and assert the raised error.
- test_on_model_error_callback_fallback_to_runner: run the agent and assert
  the unhandled model error surfaces as a terminal error event
  (error_code == 'ClientError'), mirroring the error-event assertions in
  test_base_llm_flow.py.

No production code changes.

Author: anxkhn

tests/unittests/flows/llm_flows/test_plugin_model_callbacks.py

@@ -171,18 +171,20 @@ def test_on_model_error_callback_with_plugin(mock_plugin):
 
 
 def test_on_model_error_callback_fallback_to_runner(mock_plugin):
-  """Tests that the model error is not handled and falls back to raise from runner."""
+  """Tests that the model error is not handled and surfaces from the runner."""
   mock_model = testing_utils.MockModel.create(error=mock_error, responses=[])
   mock_plugin.enable_on_model_error_callback = False
   agent = Agent(
       name='root_agent',
       model=mock_model,
   )
 
-  try:
-    testing_utils.InMemoryRunner(agent, plugins=[mock_plugin])
-  except Exception as e:
-    assert e == mock_error
+  runner = testing_utils.InMemoryRunner(agent, plugins=[mock_plugin])
+
+  events = runner.run('test')
+  error_events = [e for e in events if e.error_code]
+  assert len(error_events) == 1
+  assert error_events[0].error_code == 'ClientError'
 
 
 if __name__ == '__main__':

tests/unittests/flows/llm_flows/test_plugin_tool_callbacks.py

@@ -180,10 +180,9 @@ async def test_async_on_tool_error_fallback_to_runner(
 ):
   mock_plugin.enable_on_tool_error_callback = False
 
-  try:
+  with pytest.raises(ClientError) as exc_info:
     await invoke_tool_with_plugin(mock_error_tool, mock_plugin)
-  except Exception as e:
-    assert e == mock_error
+  assert exc_info.value == mock_error
 
 
 async def invoke_tool_with_plugin_live(
@@ -258,10 +257,9 @@ async def test_live_on_tool_error_fallback_to_runner(
 ):
   mock_plugin.enable_on_tool_error_callback = False
 
-  try:
+  with pytest.raises(ClientError) as exc_info:
     await invoke_tool_with_plugin_live(mock_error_tool, mock_plugin)
-  except Exception as e:
-    assert e == mock_error
+  assert exc_info.value == mock_error
 
 
 @pytest.mark.asyncio