Add IImageGenerator implementation for xAI image generation

[Copilot]

• Understand repository structure and build process
• Review xAI image.proto and OpenAI's IImageGenerator implementation
• Create GrokImageGenerator class implementing IImageGenerator
• Add AsIImageGenerator extension method to GrokClientExtensions
• Update GrokClientExtensions to support Image.ImageClient
• Add comprehensive tests for image generation
• Apply code style conventions (no underscore prefix, no redundant visibility, no braces for one-liners)
• Address code review feedback (GetService pattern, error handling, constants)
• Fix API compatibility (Contents vs Images, DefaultModelId vs ModelId)
• Build succeeds with all tests passing
• Run security checks (0 alerts found)
• Implementation complete

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for IImageGenerator</issue_title>
<issue_description>

<agent_instructions>Read the docs at https://docs.x.ai/docs/guides/image-generations.
Take inspiration from the implementation for OpenAI from https://github.com/dotnet/extensions/blob/main/src/Libraries/Microsoft.Extensions.AI.OpenAI</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add support for IImageGenerator #44

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[kzu]

🧪 Details on Ubuntu 24.04.3 LTS

from retest v1.1.0 on .NET 10.0.1 with 💜 by @devlooped

[kzu]

@copilot fixed the firewall rules. build currently doesn't succeed. try again now.

[Copilot]

@copilot fixed the firewall rules. build currently doesn't succeed. try again now.

Build now succeeds! Fixed API compatibility issues with Microsoft.Extensions.AI (Contents vs Images, DefaultModelId vs ModelId). All tests passing. Commit: 2aa4f92