DOC: Multimodal Message Schema for OpenAI Models - Docs + Error Improvements: include metadata property

[starmorph]

Checklist

• I added a very descriptive title to this issue.
• I included a link to the documentation page I am referring to (if applicable).

Issue with current documentation:

When creating a Multimodal message (PDF/Image) to an OpenAI chat model in the format below, our docs and error throwing is not clear that the metadata property is required. Currently, If the metadata property is not set correctly in a multimodal Base64ContentBlock message, we throw an unintuitive OpenAI error.

Improvement Suggestion

1. Update the following documentation pages to include the metadata property. Side note, the python version has a more comprehensive schema documented than JS.
   LangGraphJS MultiModal Inputs
   Langgraph PY MultiModal Inputs
2. Update the error when missing the metadata property in a Base64ContentBlock from @langchain/core/messages to say "missing required metadata property" instead of OpenAI error.

Base64ContentBlock

  // PDF Example Base64ContentBlock Message
  {
    type: "file",
    source_type: "base64",
    mime_type: "application/pdf",
    data,
    metadata: { filename: file.name },
  };
}

// Image Example Base64ContentBlock Message
{
      type: "image",
      source_type: "base64",
      mime_type: file.type,
      data,
      metadata: { name: file.name },
    };

Idea or request for content:

No response

[byteakp]

Hi 👋 Just submitted a PR that addresses this issue: #8244 .

The PR improves the documentation and examples by clearly showing that the metadata property is required when sending Base64-encoded content blocks (e.g., PDFs or images) to OpenAI chat models. It also adds caution notes and clarifies code snippets for better developer understanding.