.Net: feat(connectors): Support ImageContent in tool/function results

[Cozmopolit]

Motivation and Context

Fixes #13430

Currently, when a Semantic Kernel function returns ImageContent, it gets serialized to JSON - losing the binary image data and preventing multimodal-capable models from processing the image.

This PR enables ImageContent preservation in tool/function results, allowing connectors with multimodal capabilities (Gemini 3+, Anthropic) to pass images natively to the model. This is essential for agentic workflows where tools generate or process images that the model needs to analyze.

Description

FunctionCallsProcessor (shared infrastructure)

• Changed ProcessFunctionResult() return type from string to object
• Added early return for ImageContent to preserve it for multimodal-capable connectors
• Added ImageContentNotSupportedErrorMessage constant for consistent error messaging

Google Gemini Connector (native support)

• Extended FunctionResponsePart with Parts property for nested multimodal content
• Added FunctionResponsePartContent class with InlineData support
• Implemented CreateImageFunctionResponsePart() to convert ImageContent to Gemini's native inlineData format

OpenAI Connector (error handling)

• Added ImageContent check with clear error message (API does not support images in tool results)

OpenAI Agents (error handling)

• Added GetFunctionResultAsString() helper with ImageContent error handling

Amazon Bedrock Agents (error handling)

• Added GetFunctionResultAsString() helper with ImageContent error handling

New Unit Tests

Test	File
ItShouldPreserveImageContentWithoutSerialization	FunctionCallsProcessorTests.cs
FromChatHistoryImageContentInToolResultCreatesInlineDataPart	GeminiRequestTests.cs
FromChatHistoryImageContentWithoutDataThrowsInvalidOperationException	GeminiRequestTests.cs
FromChatHistoryImageContentWithoutMimeTypeThrowsInvalidOperationException	GeminiRequestTests.cs
VerifyAssistantMessageAdapterGetMessageWithImageContentInFunctionResult	AssistantMessageFactoryTests.cs

Notes

• No breaking changes: All changes are to internal APIs. The public FunctionResultContent.Result property is already object?.
• Gemini version detection: The connector does not check the model version. If an older Gemini model does not support functionResponse.parts, the API will return an appropriate error.
• URI-based ImageContent: Only ImageContent with binary data is supported. URI-based ImageContent will throw InvalidOperationException.
• MistralAI: Out of scope - has its own ProcessFunctionResult implementation.
• Related: Prepares infrastructure for PR .Net: feat: Add Anthropic Connector for Claude models #13419 (Anthropic Connector multimodal support)

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

sk-pr-MultimodalToolResults.md