Replace Extensions.Grok project with xAI nuget package

[Copilot]

• Delete the src/Extensions.Grok/ directory
• Delete the test file src/Tests/GrokTests.cs
• Remove Extensions.Grok project reference from solution file Extensions.AI.slnx
• Remove Extensions.Grok project reference from src/Extensions/Extensions.csproj
• Add xAI nuget package reference to src/Extensions/Extensions.csproj (moved to correct ItemGroup)
• Remove Extensions.Grok project reference from src/Tests/Tests.csproj
• Update namespace references in src/Extensions/ChatClientProviders.cs from Devlooped.Extensions.AI.Grok to xAI
• Update namespace references in src/Agents/ConfigurableAIAgent.cs from Devlooped.Extensions.AI.Grok to xAI
• Update readme.md to replace the Grok section with a reference to xAI package
• Verify the build passes
• Run code review
• Run security check (CodeQL)

Original prompt

Replace the Extensions.Grok project with the nuget package xAI which provides the same behavior now. Delete the entire project and any test files related to it.

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[kzu]

🧪 Details on Ubuntu 24.04.3 LTS

from retest v1.1.0 on .NET 10.0.2 with 💜 by @devlooped

[kzu]

Check if the xAI namespace is necessary at all in the ConfigurableAIAgent since we now use providers for the creation of the specific clients and there shouldn't be anything xAI/Grok specific there?
Also, package references should go in their own ItemGroup in MSBuild, alongside other PackageReference elements, just like ProjectReference.

[Copilot]

Check if the xAI namespace is necessary at all in the ConfigurableAIAgent since we now use providers for the creation of the specific clients and there shouldn't be anything xAI/Grok specific there?
...

The xAI namespace is still needed in ConfigurableAIAgent.cs for GrokChatOptions (lines 119-120) - it provides Grok-specific properties like Search and Include that need to be deserialized from config when the provider is "xai". Without it, users configuring Grok-specific options would lose those settings.

Moved the xAI PackageReference to the correct ItemGroup with other PackageReferences in fe62bb2.