.Net Add more tests

              We need test coverage similar to here: https://github.com/microsoft/semantic-kernel/blob/main/dotnet/src/SemanticKernel.UnitTests/PromptTemplate/KernelPromptTemplateTests.cs

In particular make sure the test cases for unsafe content are all covered

_Originally posted by @markwallace-microsoft in https://github.com/microsoft/semantic-kernel/pull/5961#discussion_r1579143773_
            