Content::Audio
The MCP spec defines an audio content type, but the SDK only had `Content::Text` and `Content::Image`. Add `Content::Audio` with the same structure as `Content::Image`. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#audio-content
Content::Image#to_h
mimeType
This is a fix for a similar bug that remained, as in #235. `Content::Image#to_h` returned `mime_type` (snake_case), but the MCP spec requires `mimeType` (camelCase) for the MIME type field in image content.