.NET: Cleanup unnecessary usages of AsIChatClient

[westey (westey-m)]

Motivation and Context

#4505

Description

• Cleanup unnecessary usages of AsIChatClient

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.

[Copilot]

Pull request overview

This PR addresses Issue #4505 by removing unnecessary .AsIChatClient() calls from sample code throughout the dotnet samples. The AsAIAgent() extension method on OpenAI.Chat.ChatClient already internally calls AsIChatClient(), so the explicit intermediate call was redundant and required an extra dependency (Microsoft.Extensions.AI.OpenAI).

Changes:

• Replaced .GetChatClient(deploymentName).AsIChatClient().AsAIAgent(...) with .GetChatClient(deploymentName).AsAIAgent(...) across all AGUI and end-to-end sample projects.
• Removed unnecessary Microsoft.Extensions.AI.OpenAI package references and using Microsoft.Extensions.AI; imports where they were only needed for the AsIChatClient() extension method.
• Updated README documentation in the AGUIWebChat sample to match the simplified code pattern.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/samples/05-end-to-end/AspNetAgentAuthorization/Service/Service.csproj	Removed Microsoft.Extensions.AI.OpenAI package reference
dotnet/samples/05-end-to-end/AspNetAgentAuthorization/Service/Program.cs	Added using OpenAI.Chat;, removed .AsIChatClient() call
dotnet/samples/05-end-to-end/AGUIWebChat/Server/Program.cs	Removed using Microsoft.Extensions.AI; and .AsIChatClient() call
dotnet/samples/05-end-to-end/AGUIWebChat/README.md	Updated two code snippets to remove .AsIChatClient()
dotnet/samples/05-end-to-end/AGUIClientServer/AGUIDojoServer/ChatClientAgentFactory.cs	Changed alias import to namespace import, removed 8 .AsIChatClient() calls
dotnet/samples/02-agents/AGUI/Step05_StateManagement/Server/Server.csproj	Removed Microsoft.Extensions.AI.OpenAI package reference
dotnet/samples/02-agents/AGUI/Step05_StateManagement/Server/Program.cs	Removed using Microsoft.Extensions.AI; and .AsIChatClient() call
dotnet/samples/02-agents/AGUI/Step04_HumanInLoop/Server/Server.csproj	Removed Microsoft.Extensions.AI.OpenAI package reference
dotnet/samples/02-agents/AGUI/Step04_HumanInLoop/Server/Program.cs	Removed .AsIChatClient() call
dotnet/samples/02-agents/AGUI/Step03_FrontendTools/Server/Server.csproj	Removed Microsoft.Extensions.AI.OpenAI package reference
dotnet/samples/02-agents/AGUI/Step03_FrontendTools/Server/Program.cs	Removed using Microsoft.Extensions.AI; and .AsIChatClient() call
dotnet/samples/02-agents/AGUI/Step02_BackendTools/Server/Server.csproj	Removed Microsoft.Extensions.AI.OpenAI package reference
dotnet/samples/02-agents/AGUI/Step02_BackendTools/Server/Program.cs	Removed .AsIChatClient() call
dotnet/samples/02-agents/AGUI/Step01_GettingStarted/Server/Server.csproj	Removed Microsoft.Extensions.AI.OpenAI package reference
dotnet/samples/02-agents/AGUI/Step01_GettingStarted/Server/Program.cs	Removed using Microsoft.Extensions.AI; and .AsIChatClient() call