fix(web): allow session resume without new message

Merge https://github.com/google/adk-python/pull/4185

**Description**
 This PR resolves #4100 by making the `new_message` field optional in the `RunAgentRequest` model.

 **The Problem:**
 When attempting to resume an agent session via the FastAPI web server, the request would fail with a `422 Unprocessable Entity` if `new_message` was omitted. This prevented "resume-only" workflows where a user just wants to wake up an existing session.

 **The Solution:**
 Updated `RunAgentRequest.new_message` to be `Optional[types.Content] = None`. The underlying `runner.run_async` logic already supports `None` for resuming purposes, so no further logic changes were required.

 **Verification:**
 Verified that `RunAgentRequest` now validates successfully when `new_message` is missing, defaulting the field to `None`.

Co-authored-by: Liang Wu <wuliang@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4185 from Akshat8510:fix/fastapi-resume-4100 b6d252636aa5f96186507fccf47a278fe733a362
PiperOrigin-RevId: 869761179

src/google/adk/cli/adk_web_server.py

@@ -204,7 +204,7 @@ class RunAgentRequest(common.BaseModel):
   app_name: str
   user_id: str
   session_id: str
-  new_message: types.Content
+  new_message: Optional[types.Content] = None
   streaming: bool = False
   state_delta: Optional[dict[str, Any]] = None
   # for resume long-running functions

tests/unittests/cli/test_fast_api.py

@@ -18,9 +18,7 @@ import logging
 import os
 from pathlib import Path
 import signal
-import sys
 import tempfile
-import time
 from typing import Any
 from typing import Optional
 from unittest.mock import AsyncMock
@@ -38,14 +36,12 @@ from google.adk.errors.input_validation_error import InputValidationError
 from google.adk.evaluation.eval_case import EvalCase
 from google.adk.evaluation.eval_case import Invocation
 from google.adk.evaluation.eval_result import EvalSetResult
-from google.adk.evaluation.eval_set import EvalSet
 from google.adk.evaluation.in_memory_eval_sets_manager import InMemoryEvalSetsManager
 from google.adk.events.event import Event
 from google.adk.events.event_actions import EventActions
 from google.adk.runners import Runner
 from google.adk.sessions.in_memory_session_service import InMemorySessionService
 from google.adk.sessions.session import Session
-from google.adk.sessions.state import State
 from google.genai import types
 from pydantic import BaseModel
 import pytest
@@ -1530,6 +1526,23 @@ def test_builder_save_rejects_traversal(builder_test_client, tmp_path):
   assert not (tmp_path / "app" / "tmp" / "escape.yaml").exists()
 
 
+def test_agent_run_resume_without_message_success(
+    test_app, create_test_session
+):
+  """Test that /run allows resuming a session with only an invocation_id, without a new message."""
+  info = create_test_session
+  url = "/run"
+  payload = {
+      "app_name": info["app_name"],
+      "user_id": info["user_id"],
+      "session_id": info["session_id"],
+      "invocation_id": "test_invocation_id",
+      "streaming": False,
+  }
+  response = test_app.post(url, json=payload)
+  assert response.status_code == 200
+
+
 def test_health_endpoint(test_app):
   """Test the health endpoint."""
   response = test_app.get("/health")