.Net: Support Support BinaryContent in Gemini Connector

[Millmer]

Motivation and Context

Resolves: #13131

Description

The Gemini connector actually already supports arbitrary base64 PDF content by prioritising binary data in the CreateGeminiPartFromImage function in the GeminiRequest.cs.

I kept the current behaviour with the ImageContent and AudioContent but also added explicit support for the BinaryContent KernelContent type in the GetGeminiPartFromKernelContent function to bring it inline with how the other connectors work.

I applied the same Chat Completion Unit test as those I found in the OpenAI Connector for BinaryContent.

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 (I think...) 😄

[Millmer]

@microsoft-github-policy-service agree

[rogerbarreto]

@Millmer please can you also add one integration test for this change, I'm happy to run that on my side and if all ok, happy to validate and merge the change.

Thanks for the contribution!