.NET: Add Google ChatClientAgent provider sample using with Gemini model

[Roger Barreto (rogerbarreto)]

Motivation and Context

• Resolves.NET sample whatif it's NOT in Azure? #1137

[Copilot]

Pull request overview

This PR adds a comprehensive sample demonstrating how to create an AIAgent using Google Gemini models as the underlying inference service. The implementation addresses issue #1137.

Key changes:

• Adds a new sample project demonstrating two different approaches to using Google Gemini models with ChatClientAgent
• Includes temporary IChatClient wrapper implementations until the official Google.GenAI library incorporates Microsoft.Extensions.AI support
• Configures package dependencies and adds the project to the solution

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
dotnet/samples/GettingStarted/AgentProviders/Agent_With_GoogleGemini/README.md	Documentation explaining the sample, prerequisites, and the two different package options for Google Gemini integration
dotnet/samples/GettingStarted/AgentProviders/Agent_With_GoogleGemini/Program.cs	Sample code demonstrating both the official Google.GenAI and community-driven Mscc.GenerativeAI.Microsoft implementations
dotnet/samples/GettingStarted/AgentProviders/Agent_With_GoogleGemini/GoogleGenAIExtensions.cs	Extension methods to convert Google.GenAI Client to IChatClient
dotnet/samples/GettingStarted/AgentProviders/Agent_With_GoogleGemini/GeminiChatClient.cs	Internal IChatClient implementation wrapping Google.GenAI functionality (temporary until official support)
dotnet/samples/GettingStarted/AgentProviders/Agent_With_GoogleGemini/Agent_With_GoogleGemini.csproj	Project configuration with Google.GenAI and Mscc.GenerativeAI.Microsoft package references
dotnet/agent-framework-dotnet.slnx	Adds the new sample project to the solution with alphabetical reordering of some existing projects
dotnet/Directory.Packages.props	Adds package version entries for Google.GenAI (0.6.0) and Mscc.GenerativeAI.Microsoft (2.9.3)