Add runtime context parameter for callbacks

[anivar]

My take on the discussion in #607

The issue raised a good point about callbacks needing access to runtime objects like A2A task updaters. I took a shot at implementing this with what seemed like the most straightforward approach.

What I built

Added a runtime_context parameter to run methods that gets passed through to callbacks:

# When running the agent, pass runtime objects
runtime_context = {"task_updater": updater, "user_session": session}
await agent.run_async("query", runtime_context=runtime_context)

Callbacks can then grab what they need:

def before_agent_invocation(self, context, prompt, **kwargs):
    if context.runtime_context and "task_updater" in context.runtime_context:
        updater = context.runtime_context["task_updater"]
        updater.update_status("started", "Processing query...")
    return context

Implementation approach

• Added an optional runtime_context field to the existing Context object
• Made it a keyword-only parameter so it doesn't mess with existing code
• Keeps it simple - just a dictionary that flows through the callback system
• Backward compatible since it defaults to None

Why this approach

Seemed like the cleanest way to solve the A2A task updater problem without over-engineering it. The Context object already flows through all callbacks, so extending it felt natural.

Happy to adjust based on feedback!

Addresses #607