ChatMessage.from_openai_dict_format wraps list-of-parts content in TextContent, breaking msg.text contract

[Vedant-Agarwal]

Describe the bug

ChatMessage.from_openai_dict_format only handles content as a plain string for user, system, and developer messages. When content is a list of content parts (the standard OpenAI Chat Completions format for multimodal messages, and also valid for text-only messages), the whole list is passed verbatim into a single TextContent. Only the tool role branch handles list content.

As a result:

• msg.text returns a list instead of str, violating the str | None contract of ChatMessage.text / TextContent.text.
• msg.to_dict() and msg.to_openai_dict_format() emit invalid content (a list nested where a string is expected), so the message cannot be round-tripped or sent back to OpenAI.
• Image/file parts are silently corrupted into "text" instead of ImageContent / FileContent.

To Reproduce

from haystack.dataclasses import ChatMessage

b64 = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mP8/x8AAwMCAO+ip1sAAAAASUVORK5CYII="
msg = ChatMessage.from_openai_dict_format({
    "role": "user",
    "content": [
        {"type": "text", "text": "What is in this image?"},
        {"type": "image_url", "image_url": {"url": f"data:image/png;base64,{b64}"}},
    ],
})

print(type(msg.text))  # <class 'list'> — should be str
print(msg.images)      # [] — the image part is lost
print(msg.to_openai_dict_format())
# {'role': 'user', 'content': [{'type': 'text', 'text': 'What is in this image?'}, ...]}
# is what you'd expect, but instead 'content' is the raw list stringified inside a TextContent,
# producing an invalid OpenAI payload on re-serialization

Expected behavior

• {"type": "text"} parts become TextContent, {"type": "image_url"} parts with base64 data URLs become ImageContent, {"type": "file"} parts with inline file_data become FileContent (the exact inverse of to_openai_dict_format, which already produces these part formats).
• msg.text returns the first text part as str.
• system/developer messages accept lists of text parts.
• Unsupported parts (non-data image URLs, file_id references, unknown types) raise ValueError instead of silently corrupting the message.

Describe your environment (please complete the following information):

• OS: macOS
• Haystack version: current main (d10abda, 3.2.0rc0); also reproducible on 2.x releases
• Python version: 3.13

I have a fix ready and will open a PR shortly.