.NET Workflows - Update changes to OpenAI to expose ResponseContentPart.InputImageUrl

### Description

Related to issue: https://github.com/openai/openai-dotnet/issues/875 and initial PR: https://github.com/openai/openai-dotnet/pull/874

This feature is tracking the work to update the proposed changes to unblock the merge process while the original PR author (Chris) is on vacation.

### Code Sample

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### Language/SDK

.NET