`McpTool` only supports returning text types, not image

[aaronpowell]

Is your feature request related to a problem? Please describe.
I'm attempting to build out a .NET implementation of server-everything and one of the features is the Tiny Image Tool which returns a combination of text and image messages (https://github.com/modelcontextprotocol/servers/blob/e328131d99847e1eae33c7a6a8156fd217cf984f/src/everything/everything.ts#L460-L479). If you implement a McpTool method you are unable to return something that can have different content types, as the return value is always serialized to a content type of text.

Describe the solution you'd like
Some way in which you can return a different content type/collection of content types, without having to fallback to implementing a CallToolHandler.

Describe alternatives you've considered
Implementing as a CallToolHandler.

Additional context
Sample: https://github.com/aaronpowell/modelcontextprotocol-csharp-sdk/blob/dotnet-everything/samples/EverythingServer/Tools/TinyImageTool.cs

[aaronpowell]

Just checking that this is now supported, as I couldn't grasp from #89 how to return different stuff from McpServerTool - is it a case of creating a Content object and returning that?

[stephentoub]

You can return a DataContent:

csharp-sdk/src/ModelContextProtocol/Server/AIFunctionMcpServerTool.cs

Lines 198 to 207 in 4b70bc7

	case DataContent dataContent:
	return new()
	{
	Content = [new()
	{
	Data = dataContent.GetBase64Data(),
	MimeType = dataContent.MediaType,
	Type = dataContent.HasTopLevelMediaType("image") ? "image" : "resource",
	}]
	};

and it'll be mapped either to "image" or "resource" based on its mime type.

We can augment this list with other types as well. I believe that's what #41 is tracking. Content might be a good candidate.