404 Not Found - ADK Fastapi - only chatbot works - we need Pre-defined buttons for predefined prompts

[RaffaelloRalf]

Describe the bug
I try to replace the chatbot function in the Gemini Fullstack sample with predefined buttons in order to remove the chat functionality
https://github.com/google/adk-samples/tree/main/python/agents/gemini-fullstack

Whatever we try to change, we get a 404 Not Found error, when the frontend sends the request to the ADK Fastapi backend via the predefined buttons.
As far as we are aware, the EventSource in the browser makes a GET request by default, but the predefined FastAPI
endpoint is defined to only accept POST requests. The EventSource API does not support POST requests
directly.

We would like to provide a predefined resesarch plan to the "Research Pipeline" instead of using the Interactive Planner Agent

A clear and concise description of what the bug is.

Steps to reproduce the behavior:

1. take the gemini-fullstack template
2. change the chat.tsx and implement predefied buttons
3. --> 404 error. the adk seems only to handle chat bot requests.

Expected behavior
the ADK accepts the requests and the root agent reads the predefined plan from the backend and starts the research

Desktop (please complete the following information):

• firebase studio
• ADK version(pip show google-adk): every one

Model Information:
2.5 pro

Additional context
"POST /apps/app/users/u_999/sessions/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxHTTP/1.1" 200 OK
INFO: 127.x.x.x:xxxx - "POST /run_workflow/.xxxx HTTP/1.1" 404 Not Found