.Net: Start to add integration tests

[markwallace-microsoft]

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Pull Request Overview

This PR introduces integration tests for the OpenAI response agents and updates some agent test configurations for improved flexibility and error handling.

• Updated BaseResponsesAgentTest to use a configuration-based model id.
• Added multiple integration test files for verifying OpenAIResponseAgent behavior and thread conformance.
• Updated OpenAIResponseAgentThread to expose a public CreateAsync method and improve deletion error handling.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/InternalUtilities/samples/AgentUtilities/BaseResponsesAgentTest.cs	Replaced hardcoded model string with configuration value.
dotnet/src/IntegrationTests/Agents/OpenAIResponseAgentTests.cs	Added integration tests for agent invocation and streaming responses.
dotnet/src/IntegrationTests/Agents/CommonInterfaceConformance/OpenAIResponsesAgentFixture.cs	Introduced a fixture to centralize test setup and teardown for OpenAI agent tests.
dotnet/src/IntegrationTests/Agents/CommonInterfaceConformance/AgentThreadConformance/OpenAIResponseAgentThreadTests.cs	Added tests focused on agent thread behavior conformance.
dotnet/src/Agents/OpenAI/OpenAIResponseAgentThread.cs	Added a public CreateAsync method and improved error handling during deletion with a try/catch block.
dotnet/src/Agents/OpenAI/OpenAIResponseAgentInvokeOptions.cs	Added a new class to encapsulate extra invocation options for the agent.