Describe the bug

As per official documentation, we should be able to modify the final agent response by operating at after_agent_callback level.

As per ADK 1.5, however, it seems there is no way to publicly access the agent's final output from within the callback_context parameter. This clearly is either a BUG in the code or an error in the documentation.

To Reproduce
Steps to reproduce the behavior:

1. Create an agent
2. Register a after_agent_callback
3. Debug and inspect the values of callback_context variable

Expected behavior
There should a way to access the final output of the agent via the callback_context or via another parameter passed to the callback.

Screenshots

Desktop (please complete the following information):

• OS: Windows 11
• Python version(python -V): 3.13
• ADK version(pip show google-adk): 1.5

Model Information:

Gemini 2.5-flash

Additional context

Possibly related to #1231. However, in my opinion, this has to be considered a BUG and not a feature request, unless we clearly change the documentation.