Fix ResponseContentPart kind setter

[fallintoplace]

Summary

• fix ResponseContentPart.Kind to use the assigned value when updating InternalType
• add a smoke regression that exercises the private setter and verifies the backing type is updated

Notes

• the current ResponseContentPart JSON deserialization path appears to set InternalType directly through concrete internal content types, so this looks like a latent correctness fix rather than a confirmed live deserialization failure
• I was not able to run the test suite locally because the dotnet SDK is not installed in this environment