AttributeError when using the GlobalInstructionPlugin with a Runner

[henrywang0314]

Describe the bug

When using the GlobalInstructionPlugin with a Runner, an AttributeError is raised during the before_model_callback. The error message, 'CallbackContext' object has no attribute 'invocation_context', indicates that the plugin is trying to access an attribute that does not exist on the context object provided by the runner's execution flow.

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To Reproduce

Steps to reproduce the behavior:

1. Initialize a Runner and include GlobalInstructionPlugin in the plugins list.
2. Call the runner.run_async() method to start an agent execution.
3. The following error occurs.

Code:

# Simplified reproduction code based on provided snippets
from google.adk.runners import Runner
from google.adk.plugins import GlobalInstructionPlugin
# ... other necessary imports for agent, services, etc.

async def get_runner_async():
    """Returns a runner configured with the GlobalInstructionPlugin."""
    # Assume root_agent and services are defined elsewhere
    root_agent = ... 
    artifact_service = ...
    session_service = ...
    memory_service = ...
    
    runner = Runner(
        app_name="my_app",
        agent=root_agent,
        artifact_service=artifact_service,
        session_service=session_service,
        memory_service=memory_service,
        plugins=[
            GlobalInstructionPlugin(
                global_instruction="This is a global instruction."
            )
        ]
    )
    return runner

async def main():
    """Main execution function to demonstrate the bug."""
    runner = await get_runner_async()
    
    # The error occurs when run_async is called and the plugin's callback is triggered
    async for event in runner.run_async(
        user_id="test_user",
        session_id="test_session",
        new_message="Hello, agent!",
    ):
        print(event)

# To run:
# import asyncio
# asyncio.run(main())