.NET: Add .NET Anthropic Claude Skills sample

[Copilot]

Motivation and Context

Adds a sample demonstrating Anthropic Claude Skills support in .NET, matching the Python sample functionality as requested in Issue #2690. Follows Stephen Toub's recommended simplified approach using AsAITool().

Description

Sample Agent_Anthropic_Step04_UsingSkills demonstrates:

• Listing available Anthropic-managed skills
• Using BetaSkillParams.AsAITool() to add skills (SDK auto-handles beta flags)
• Extended thinking configuration
• Downloading generated files via Files API

BetaSkillParams pptxSkill = new()
{
    Type = BetaSkillParamsType.Anthropic,
    SkillID = "pptx",
    Version = "latest"
};

ChatClientAgent agent = anthropicClient.Beta.AsAIAgent(
    model: model,
    instructions: "You are a helpful agent for creating PowerPoint presentations.",
    tools: [pptxSkill.AsAITool()]);

Key: AsAITool() auto-handles beta headers and container config - no manual RawRepresentationFactory needed!

Changes:

• Program.cs - Simplified with skills listing and file download
• README.md - Updated documentation
• AnthropicSkillsIntegrationTests.cs - Simplified tests

Contribution Checklist

• The code builds clean without any errors or warnings

• The PR follows the Contribution Guidelines

• All unit tests pass, and I have added new tests where possible

• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

• Fixes .NET: Claude Skills support #2690

[Copilot]

Pull request overview

This PR adds a new .NET sample demonstrating Anthropic Claude Skills support, matching Python sample functionality as requested in Issue #2690. The implementation uses a simplified AsAITool() approach that automatically handles beta flags and container configuration.

Changes:

• Added a new sample Agent_Anthropic_Step04_UsingSkills demonstrating Skills usage (pptx skill)
• Added integration tests for Skills functionality in AnthropicSkillsIntegrationTests.cs
• Updated parent README to reference the new sample

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
AnthropicSkillsIntegrationTests.cs	Integration tests for Anthropic Skills functionality with two test cases
Agent_Anthropic_Step04_UsingSkills/README.md	Documentation for the Skills sample with code examples and prerequisites
Agent_Anthropic_Step04_UsingSkills/Program.cs	Sample code demonstrating Skills listing, agent creation, and file download
Agent_Anthropic_Step04_UsingSkills.csproj	Project configuration file matching existing sample patterns
agent-framework-dotnet.slnx	Solution file updated to include the new sample project
AgentWithAnthropic/README.md	Updated table of samples to reference the new Skills sample