Add some OpenAI and Foundry extension methods

[markwallace-microsoft]

Motivation and Context

Proposed extension methods to make it easier to create an AIAgent instance from an OpenAIClient or PersistentAgentsClient.

Description

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
• I didn't break anyone 😄

[Copilot]

Pull Request Overview

This PR adds extension methods to simplify creating AI agents from OpenAI clients and Azure AI Foundry persistent agents, improving developer experience by providing convenient factory methods.

• Adds OpenAI client extension methods for creating AI agents with both Chat Completion and Response APIs
• Refactors Azure AI persistent agent methods with better naming and consolidated creation workflow
• Updates sample code to demonstrate the new extension methods and agent patterns

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
PersistentAgentsClientExtensions.cs	Renames GetRunnableAgentAsync to GetAIAgentAsync and adds CreateAIAgentAsync method
ChatClientAgent_With_OpenAIClient.cs	New sample demonstrating OpenAI client extension methods with multiple API patterns
ChatClientAgent_With_OpenAIChatCompletion.cs	Removed file - functionality moved to new OpenAI client sample
ChatClientAgent_With_AzureOpenAIChatCompletion.cs	Updates variable type declaration for consistency
ChatClientAgent_With_AzureAIAgentsPersistent.cs	Refactored to use new CreateAIAgentAsync method and simplified workflow
OpenAIClientExtensions.cs	New extension methods for creating AI agents from OpenAI clients and chat clients
OpenAIAgentExtensions.cs	Internal extensions for OpenAI message conversion and agent interaction
AgentRunResponseExtensions.cs	Extensions to extract native OpenAI response objects from agent responses

Comments suppressed due to low confidence (1)

dotnet/src/Microsoft.Extensions.AI.Agents.AzureAI/PersistentAgentsClientExtensions.cs:84

• The 'description' parameter is not being passed to the CreateAgentAsync method call. This will cause the description to be ignored when creating the agent.

            instructions,