Image generation tool

[ericstj]

Fixes #6639

This adds a prototype implementation of ImageGenerationTool with support in OpenAI provider.

To use this requires using the OpenAI Responses chat client with OpenAI provider (not Azure OpenAI).

Still required:

• Add tests for tool (currently testing with a sample)
• Polish and document API
• Refine implementation in OpenAI to ensure its minimally fragile (despite usage of reflection) - test with AOT
• Refine content update coalescing methods after testing in sample

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR adds a prototype implementation of ImageGenerationTool with support in the OpenAI provider, enabling AI services to perform image generation when specified as a tool.

• Introduces ImageGenerationTool as a marker tool class that can be configured with ImageGenerationOptions
• Adds experimental APIs for applying chat response updates to existing responses with configurable coalescing options
• Implements reflection-based handling of OpenAI's internal image generation response types in the OpenAI provider

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
ImageGenerationTool.cs	New tool class for enabling image generation capabilities
ChatResponseUpdateCoalescingOptions.cs	Configuration options for how response updates are merged
ChatResponseExtensions.cs	Extension methods for applying updates to chat responses
OpenAIResponsesChatClient.cs	OpenAI provider implementation with image generation support via reflection
OpenAIJsonContext.cs	Added JSON serialization support for new types
ChatResponseUpdate.cs	Fixed documentation reference
ChatResponseUpdateExtensionsTests.cs	Comprehensive tests for new update coalescing functionality

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