.Net Bug: Google Connector with Nullable Enum in Plugin Call - Bad Request

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Describe the bug
I was troubleshooting a scenario where I was getting bad request responses from my custom Semantic Kernel plugin when using the Google connector. The plugin works fine with Open AI. One of the arguments on the method is a nullable enum type. If I changed this to no longer be nullable. E.g. instead of a RemoteWork? enum I just use RemoteWork the function call goes through fine.

To Reproduce
Steps to reproduce the behavior:

1. Create a simple plugin with an enum type as a method argument, and allow it to be null (with the default value of null)
2. Have the Google LLM call the plugin in a chat connector session (using auto function choice, and auto invoke kernel functions)
3. When the function is called the call to await chatCompletionService.GetChatMessageContentAsync from the IChatCompletionService implementation will fail with a 400 bad request. Looking at the response content from Google you get the below error.

{
"error": {
"code": 400,
"message": "* GenerateContentRequest.tools[0].function_declarations[0].parameters.properties[remoteWork].enum: only allowed for STRING type\n* GenerateContentRequest.tools[0].function_declarations[0].parameters.properties[remoteWork].type: must be specified when not using one_of, any_of or all_of fields\n",
"status": "INVALID_ARGUMENT"
}
}

Expected behavior
You should be able to pass in nullable enum parameters and have them parsed correctly by the plugin.

Platform

• Language: C#
• Source: Microsoft.SemanticKernel.Connectors.Google 1.45.0-alpha
• AI model: gemini-2.5-pro-preview-03-25 and gemini-2.0-flash
• IDE: Jetbrains Rider
• OS: Mac

Additional context
Was initially discussed in the Azure AI Community Discord - conversation starts here.

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Also, the initial fix for this was made here. It probably just needs to be further accommodated for nullable types.