.NET: Simplify store=false scenario for responses

[westey-m]

Motivation and Context

#1118

Description

• Simplify creation of agents using store=false with responses

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 simplifies the creation of agents using store=false with the OpenAI Responses API, addressing issue #1118. Previously, developers had to use the "break glass" approach via RawRepresentationFactory on ChatOptions, which was verbose and non-intuitive. The new AsIChatClientWithStoredOutputDisabled() extension method provides a clean, fluent API that encapsulates this complexity.

Changes:

• Added AsIChatClientWithStoredOutputDisabled() extension method to simplify creating responses-based agents with store=false
• Added comprehensive unit tests for the new extension method
• Added sample code demonstrating the simplified API usage

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIResponseClientExtensions.cs	Adds new extension method AsIChatClientWithStoredOutputDisabled() that wraps the complex RawRepresentationFactory configuration
dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIResponseClientExtensionsTests.cs	Adds unit tests for null argument validation and service chain accessibility
dotnet/samples/GettingStarted/AgentProviders/Agent_With_AzureOpenAIResponses/Program.cs	Demonstrates the new simplified API for creating agents with store=false