openai[patch]: format system content blocks for Responses API (#31096)

```python
from langchain_core.messages import HumanMessage, SystemMessage
from langchain_openai import ChatOpenAI


llm = ChatOpenAI(model="gpt-4.1", use_responses_api=True)

messages = [
    SystemMessage("test"),                                   # Works
    HumanMessage("test"),                                    # Works
    SystemMessage([{"type": "text", "text": "test"}]),       # Bug in this case
    HumanMessage([{"type": "text", "text": "test"}]),        # Works
    SystemMessage([{"type": "input_text", "text": "test"}])  # Works
]

llm._get_request_payload(messages)
```

libs/partners/openai/langchain_openai/chat_models/base.py

@@ -3191,7 +3191,7 @@ def _construct_responses_api_input(messages: Sequence[BaseMessage]) -> list:
                 input_.append(msg)
             input_.extend(function_calls)
             input_.extend(computer_calls)
-        elif msg["role"] == "user":
+        elif msg["role"] in ("user", "system", "developer"):
             if isinstance(msg["content"], list):
                 new_blocks = []
                 for block in msg["content"]:

libs/partners/openai/tests/unit_tests/chat_models/test_base.py

@@ -956,13 +956,17 @@ def test__get_request_payload() -> None:
     messages: list = [
         SystemMessage("hello"),
         SystemMessage("bye", additional_kwargs={"__openai_role__": "developer"}),
+        SystemMessage(content=[{"type": "text", "text": "hello!"}]),
         {"role": "human", "content": "how are you"},
+        {"role": "user", "content": [{"type": "text", "text": "feeling today"}]},
     ]
     expected = {
         "messages": [
             {"role": "system", "content": "hello"},
             {"role": "developer", "content": "bye"},
+            {"role": "system", "content": [{"type": "text", "text": "hello!"}]},
             {"role": "user", "content": "how are you"},
+            {"role": "user", "content": [{"type": "text", "text": "feeling today"}]},
         ],
         "model": "gpt-4o-2024-08-06",
         "stream": False,
@@ -977,7 +981,9 @@ def test__get_request_payload() -> None:
         "messages": [
             {"role": "developer", "content": "hello"},
             {"role": "developer", "content": "bye"},
+            {"role": "developer", "content": [{"type": "text", "text": "hello!"}]},
             {"role": "user", "content": "how are you"},
+            {"role": "user", "content": [{"type": "text", "text": "feeling today"}]},
         ],
         "model": "o3-mini",
         "stream": False,
@@ -1658,6 +1664,9 @@ def test__construct_responses_api_input_multiple_message_types() -> None:
     """Test conversion of a conversation with multiple message types."""
     messages = [
         SystemMessage(content="You are a helpful assistant."),
+        SystemMessage(
+            content=[{"type": "text", "text": "You are a very helpful assistant!"}]
+        ),
         HumanMessage(content="What's the weather in San Francisco?"),
         HumanMessage(
             content=[{"type": "text", "text": "What's the weather in San Francisco?"}]
@@ -1698,31 +1707,36 @@ def test__construct_responses_api_input_multiple_message_types() -> None:
     assert result[0]["role"] == "system"
     assert result[0]["content"] == "You are a helpful assistant."
 
+    assert result[1]["role"] == "system"
+    assert result[1]["content"] == [
+        {"type": "input_text", "text": "You are a very helpful assistant!"}
+    ]
+
     # Check human message
-    assert result[1]["role"] == "user"
-    assert result[1]["content"] == "What's the weather in San Francisco?"
     assert result[2]["role"] == "user"
-    assert result[2]["content"] == [
+    assert result[2]["content"] == "What's the weather in San Francisco?"
+    assert result[3]["role"] == "user"
+    assert result[3]["content"] == [
         {"type": "input_text", "text": "What's the weather in San Francisco?"}
     ]
 
     # Check function call
-    assert result[3]["type"] == "function_call"
+    assert result[4]["type"] == "function_call"
-    assert result[3]["name"] == "get_weather"
+    assert result[4]["name"] == "get_weather"
-    assert result[3]["arguments"] == '{"location": "San Francisco"}'
+    assert result[4]["arguments"] == '{"location": "San Francisco"}'
-    assert result[3]["call_id"] == "call_123"
+    assert result[4]["call_id"] == "call_123"
-    assert result[3]["id"] == "func_456"
+    assert result[4]["id"] == "func_456"
 
     # Check function call output
-    assert result[4]["type"] == "function_call_output"
+    assert result[5]["type"] == "function_call_output"
-    assert result[4]["output"] == '{"temperature": 72, "conditions": "sunny"}'
+    assert result[5]["output"] == '{"temperature": 72, "conditions": "sunny"}'
-    assert result[4]["call_id"] == "call_123"
+    assert result[5]["call_id"] == "call_123"
-
-    assert result[5]["role"] == "assistant"
-    assert result[5]["content"] == "The weather in San Francisco is 72°F and sunny."
 
     assert result[6]["role"] == "assistant"
-    assert result[6]["content"] == [
+    assert result[6]["content"] == "The weather in San Francisco is 72°F and sunny."
+
+    assert result[7]["role"] == "assistant"
+    assert result[7]["content"] == [
         {
             "type": "output_text",
             "text": "The weather in San Francisco is 72°F and sunny.",
@@ -1733,6 +1747,25 @@ def test__construct_responses_api_input_multiple_message_types() -> None:
     # assert no mutation has occurred
     assert messages_copy == messages
 
+    # Test dict messages
+    llm = ChatOpenAI(model="o4-mini", use_responses_api=True)
+    message_dicts: list = [
+        {"role": "developer", "content": "This is a developer message."},
+        {
+            "role": "developer",
+            "content": [{"type": "text", "text": "This is a developer message!"}],
+        },
+    ]
+    payload = llm._get_request_payload(message_dicts)
+    result = payload["input"]
+    assert len(result) == 2
+    assert result[0]["role"] == "developer"
+    assert result[0]["content"] == "This is a developer message."
+    assert result[1]["role"] == "developer"
+    assert result[1]["content"] == [
+        {"type": "input_text", "text": "This is a developer message!"}
+    ]
+
 
 def test_service_tier() -> None:
     llm = ChatOpenAI(model="o4-mini", service_tier="flex")