feat: Add integration tests for litellm with and without turn on add_function_to_prompt

Add experiments for https://github.com/google/adk-python/issues/1273

PiperOrigin-RevId: 771145715

tests/integration/models/test_litellm_no_function.py

@@ -0,0 +1,65 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from google.adk.models import LlmRequest
+from google.adk.models import LlmResponse
+from google.adk.models.lite_llm import LiteLlm
+from google.genai import types
+from google.genai.types import Content
+from google.genai.types import Part
+import pytest
+
+_TEST_MODEL_NAME = "vertex_ai/meta/llama-4-maverick-17b-128e-instruct-maas"
+
+
+_SYSTEM_PROMPT = """You are a helpful assistant."""
+
+
+@pytest.fixture
+def oss_llm():
+  return LiteLlm(model=_TEST_MODEL_NAME)
+
+
+@pytest.fixture
+def llm_request():
+  return LlmRequest(
+      model=_TEST_MODEL_NAME,
+      contents=[Content(role="user", parts=[Part.from_text(text="hello")])],
+      config=types.GenerateContentConfig(
+          temperature=0.1,
+          response_modalities=[types.Modality.TEXT],
+          system_instruction=_SYSTEM_PROMPT,
+      ),
+  )
+
+
+@pytest.mark.asyncio
+async def test_generate_content_async(oss_llm, llm_request):
+  async for response in oss_llm.generate_content_async(llm_request):
+    assert isinstance(response, LlmResponse)
+    assert response.content.parts[0].text
+
+
+# Note that, this test disabled streaming because streaming is not supported
+# properly in the current test model for now.
+@pytest.mark.asyncio
+async def test_generate_content_async_stream(oss_llm, llm_request):
+  responses = [
+      resp
+      async for resp in oss_llm.generate_content_async(
+          llm_request, stream=False
+      )
+  ]
+  part = responses[0].content.parts[0]
+  assert len(part.text) > 0