.Net: Bug: ImageContent in tool/function results is serialized to JSON instead of native format

[Cozmopolit]

Describe the bug

When a kernel function returns ImageContent, FunctionCallsProcessor.ProcessFunctionResult() serializes it to JSON. This prevents connectors from using native image formats in tool results, even when the underlying API supports it (Gemini 3+, Anthropic Claude).

The model receives "{ \"Data\": \"base64...\", \"MimeType\": \"image/png\" }" as text instead of an actual image it can "see".

To Reproduce

1. Create a kernel function that returns ImageContent:

   [KernelFunction]
   public ImageContent CaptureScreenshot()
   {
       var imageBytes = TakeScreenshot();
       return new ImageContent(imageBytes, "image/png");
   }

2. Enable auto function calling with Gemini 3 connector

3. Inspect the API request - observe that functionResponse.response contains serialized JSON text instead of functionResponse.parts[].inlineData

Expected behavior

Connectors that support images in tool results should receive the original ImageContent object and convert it to their native format:

• Gemini 3+: functionResponse.parts[].inlineData
• Anthropic: ImageBlockParam in ToolResultBlockParam.Content
• OpenAI/others: Clear error message (API does not support images in tool results)

Platform

• Language: C#
• Source: main branch
• AI model: Google Gemini 3 (gemini-2.5-pro, gemini-2.5-flash)
• IDE: any
• OS: any

Additional context

API	Image Tool Results	Native Format
Google Gemini 3+	✅ Supported	functionResponse.parts[].inlineData
Anthropic Claude	✅ Supported	ImageBlockParam in ToolResultBlockParam.Content
Google Gemini 2.5 and earlier	❌ Not supported	-
OpenAI	❌ Not supported	-

Related: PR #13419 (Anthropic Connector multimodal support)