.Net: Ignore unknown response item types rather than throw

[markwallace-microsoft]

Motivation and Context

Closes #12511

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 😄

[markwallace-microsoft]

Unrelated integration test failure:

[xUnit.net 00:00:11.46]     SemanticKernel.IntegrationTests.Connectors.AzureOpenAI.AzureOpenAITextToImageTests.GetImageContentsCanReturnImageUrlAsync [FAIL]
[xUnit.net 00:00:11.46]       Microsoft.SemanticKernel.HttpOperationException : HTTP 400 (invalid_request_error: content_policy_violation)
[xUnit.net 00:00:11.46]       
[xUnit.net 00:00:11.46]       This request has been blocked by our content filters.
[xUnit.net 00:00:11.46]       ---- System.ClientModel.ClientResultException : HTTP 400 (invalid_request_error: content_policy_violation)
[xUnit.net 00:00:11.46]       
[xUnit.net 00:00:11.46]       This request has been blocked by our content filters.