.NET: Add support for getting and creating Assistant and Foundry agents with ChatClientAgentOptions

[westey-m]

Motivation and Context

#1226

We don't have extensions to consistently get and create Foundry Agents and Assistants using ChatClientAgentOptions, which means that adding chat message store factories or ai context provider factories is not always possible.

Description

• Add missing get and create overloads with ChatClientAgentOptions.

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 adds support for getting and creating Assistant and Foundry agents using ChatClientAgentOptions, addressing the missing extension methods that prevented consistent use of chat message store factories and AI context provider factories.

• Add new overloads for GetAIAgent methods that accept ChatClientAgentOptions parameters
• Add new overloads for CreateAIAgent methods that accept ChatClientAgentOptions parameters
• Update existing CreateAIAgent methods to properly copy all ChatClientAgentOptions properties

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIAssistantClientExtensions.cs	Added GetAIAgent and CreateAIAgent overloads with ChatClientAgentOptions support
dotnet/src/Microsoft.Agents.AI.AzureAI/PersistentAgentsClientExtensions.cs	Added GetAIAgent and CreateAIAgent overloads with ChatClientAgentOptions support
dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIAssistantClientExtensionsTests.cs	Added comprehensive unit tests for new GetAIAgent overloads
dotnet/tests/Microsoft.Agents.AI.AzureAI.UnitTests/Extensions/PersistentAgentsClientExtensionsTests.cs	Added comprehensive unit tests for new GetAIAgent and CreateAIAgent overloads
dotnet/.vscode/settings.json	Updated VS Code settings to enable chat agent functionality

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