Prompt enum values should be serialized as all lowercase

[Redth]

Describe the bug
When returning a prompt with a prompt message like the following:

new PromptMessage {
	Role = Role.Assistant,
	Content = new Content	 {
		Text = "some content",
		Type = "text",
	}
}

The JSON seems to be serialized with "role": "Assistant". Some MCP clients appear to be picky about the casing of this value. Claude is complaining:

Expected behavior
The role enum value should be lowercase when serialized.

[stephentoub]

Already fixed by #61?