fix: bug which causes unrecoverable state if previous message was an image (#6486)

* trying tout gpt-5.1-codex

* add unit test for message content

* try to support multimodal

tests/test_message_serialization.py

@@ -0,0 +1,32 @@
+from letta.llm_api.openai_client import fill_image_content_in_responses_input
+from letta.schemas.enums import MessageRole
+from letta.schemas.letta_message_content import Base64Image, ImageContent, TextContent
+from letta.schemas.message import Message
+
+
+def _user_message_with_image_first(text: str) -> Message:
+    image = ImageContent(source=Base64Image(media_type="image/png", data="dGVzdA=="))
+    return Message(role=MessageRole.user, content=[image, TextContent(text=text)])
+
+
+def test_to_openai_responses_dicts_handles_image_first_content():
+    message = _user_message_with_image_first("hello world")
+    serialized = Message.to_openai_responses_dicts_from_list([message])
+    parts = serialized[0]["content"]
+    assert any(part["type"] == "input_text" and part["text"] == "hello world" for part in parts)
+    assert any(part["type"] == "input_image" for part in parts)
+
+
+def test_fill_image_content_in_responses_input_includes_image_parts():
+    message = _user_message_with_image_first("describe image")
+    serialized = Message.to_openai_responses_dicts_from_list([message])
+    rewritten = fill_image_content_in_responses_input(serialized, [message])
+    assert rewritten == serialized
+
+
+def test_to_openai_responses_dicts_handles_image_only_content():
+    image = ImageContent(source=Base64Image(media_type="image/png", data="dGVzdA=="))
+    message = Message(role=MessageRole.user, content=[image])
+    serialized = Message.to_openai_responses_dicts_from_list([message])
+    parts = serialized[0]["content"]
+    assert parts[0]["type"] == "input_image"