.NET: fix forwarding conversationId to IChatClient in OpenAI Responses

[Korolev Dmitry (DeagleGross)]

In #2477 the problem was raised: client's conversationId does not map to what IChatClient expects in the options.

We should not forward the client's conversationId into IChatClient.

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 an issue where the client's conversationId was incorrectly being forwarded to the underlying IChatClient via ChatOptions.ConversationId. The fix ensures that conversation management remains at the hosting layer level and is not passed down to the chat client, which may have its own conversation handling or none at all.

Key changes:

• Removed forwarding of ConversationId from request to ChatOptions in response executors
• Added comprehensive tests to verify the fix works for both streaming and non-streaming scenarios
• Enhanced test infrastructure to track ChatOptions passed to mock chat clients

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/AIAgentResponseExecutor.cs	Removed ConversationId assignment to ChatOptions with explanatory comment
dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/HostedAgentResponseExecutor.cs	Removed ConversationId assignment to ChatOptions with explanatory comment
dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/TestHelpers.cs	Added LastChatOptions property to SimpleMockChatClient for test verification
dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIResponsesIntegrationTests.cs	Added two integration tests and helper methods to verify conversationId is not forwarded to IChatClient