Add test for file upload and inline file handling

[DracoBlue]

We need tests covering the upload of a file like this:

{
  "jsonrpc": "2.0",
  "id": "req-007",
  "method": "message/send",
  "params": {
    "message": {
      "role": "user",
      "parts": [
        {
          "kind": "text",
          "text": "Analyze this image and highlight any faces."
        },
        {
          "kind": "file",
          "file": {
            "name": "input_image.png",
            "mimeType": "image/png",
            "bytes": "iVBORw0KGgoAAAANSUhEUgAAAAUA..." // Base64 encoded image data
          }
        }
      ],
      "messageId": "6dbc13b5-bd57-4c2b-b503-24e381b6c8d6"
    }
  }
}

transformed from attachments like in https://ai-sdk.dev/cookbook/node/stream-text-with-file-prompt#stream-text-with-file-prompt

  const result = streamText({
    model: anthropic('claude-3-5-sonnet-20241022'),
    messages: [
      {
        role: 'user',
        content: [
          {
            type: 'text',
            text: 'What is an embedding model according to this document?',
          },
          {
            type: 'file',
            data: fs.readFileSync('./data/ai.pdf'),
            mimeType: 'application/pdf',
          },
        ],
      },
    ],
  });