Impossible to resume agents throught fastapi endpoint without "new_message"

[Armadindon]

Describe the bug
Unable to resume agents without specifying "new_message"

To Reproduce

1. Create a new session

> curl -X POST -H "Content-Type: application/json" http://localhost:8000/apps/my_app/users/user/sessions
{"id":"a0eccf39-5499-4990-8b09-14a336a3c14e","appName":"my_app","userId":"user","state":{},"events":[],"lastUpdateTime":1767879087.399263}

2. Start an invocation

> curl -X POST -H "Content-Type: application/json" -H "Accept: text/event-stream" -H "Cache-Control: no-cache" http://localhost:8000/run_sse --data '{"app_name": "my_app", "user_id": "user", "session_id": "a0eccf39-5499-4990-8b09-14a336a3c14e", "new_message": {"role": "user", "parts": [{"text": "Hello !"}]}}'
...
data: {...,"invocationId":"e-8679c93c-6a64-4669-a35a-d05c018cd40a", ...}

3. Cut the server during the invocation
4. Restart the adk server
5. Try to resume the invocation using

> curl -X POST -H "Content-Type: application/json" -H "Accept: text/event-stream" -H "Cache-Control: no-cache" http://localhost:8000/run_sse --data '{"app_name": "my_app", "user_id": "user", "session_id": "a0eccf39-5499-4990-8b09-14a336a3c14e", "invocation_id": "e-8679c93c-6a64-4669-a35a-d05c018cd40a"}'
{"detail":[{"type":"missing","loc":["body","newMessage"],"msg":"Field required","input":{"app_name":"my_app","user_id":"user","session_id":"a0eccf39-5499-4990-8b09-14a336a3c14e","invocation_id":"e-8679c93c-6a64-4669-a35a-d05c018cd40a"}}]}% 

Expected behavior
Should resume invocation with given id as stated in the documentation: https://google.github.io/adk-docs/runtime/resume/#resume-a-stopped-workflow.

Desktop (please complete the following information):

• OS: Linux
• Python version(python -V): 3.10.12
• ADK version(pip show google-adk): 1.21.0

Additional context
I think we only need to make Optional "newMessage" in the pydantic model here:

adk-python/src/google/adk/cli/adk_web_server.py

Line 205 in a0b2aac

new_message: types.Content

[Akshat8510]

I'd like to help fix this FastAPI session resume issue. I've been working on the ADK adapters and would like to ensure the session state flow is working correctly for web deployments. Can I take this on?