fix: Update LiteLLM system instruction role from "developer" to "system"

This change replaces the use of `ChatCompletionDeveloperMessage` with `ChatCompletionSystemMessage` and sets the role to "system" for providing system instructions to LiteLLM models

Close #3657

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 835388738

Author: GWeale

src/google/adk/models/lite_llm.py

@@ -39,8 +39,8 @@
 from litellm import acompletion
 from litellm import ChatCompletionAssistantMessage
 from litellm import ChatCompletionAssistantToolCall
-from litellm import ChatCompletionDeveloperMessage
 from litellm import ChatCompletionMessageToolCall
+from litellm import ChatCompletionSystemMessage
 from litellm import ChatCompletionToolMessage
 from litellm import ChatCompletionUserMessage
 from litellm import completion
@@ -983,8 +983,8 @@ def _get_completion_inputs(
   if llm_request.config.system_instruction:
     messages.insert(
         0,
-        ChatCompletionDeveloperMessage(
-            role="developer",
+        ChatCompletionSystemMessage(
+            role="system",
             content=llm_request.config.system_instruction,
         ),
     )

tests/unittests/models/test_litellm.py

@@ -1195,7 +1195,7 @@ async def test_generate_content_async_with_system_instruction(
 
   _, kwargs = mock_acompletion.call_args
   assert kwargs["model"] == "test_model"
-  assert kwargs["messages"][0]["role"] == "developer"
+  assert kwargs["messages"][0]["role"] == "system"
   assert kwargs["messages"][0]["content"] == "Test system instruction"
   assert kwargs["messages"][1]["role"] == "user"
   assert kwargs["messages"][1]["content"] == "Test prompt"