[Doc] before_agent_callback incorrectly assumes that invocation_context can be modified.

Under https://google.github.io/adk-docs/callbacks/types-of-callbacks/#before-agent-callback:

It specifies that "Before Agent Callback" is "Ideal for setting up resources or state needed only for this specific agent's run, performing validation checks on the session state (callback_context.state) before execution starts, logging the entry point of the agent's activity, or **potentially modifying the invocation context before the core logic uses it.**"

However, SingleBeforeModelCallback is typed with CallbackContext which is ReadOnly.

Should the documentation be updated is there something I'm missing with the before agent callback method?