Fix Resource::Contents#to_h to use correct property names per MCP spec

This PR fixes thr following fields with the MCP specification which requires:

- Text resources use `text` and `mimeType` fields
- Binary resources use `blob` (base64-encoded) and `mimeType` fields

https://modelcontextprotocol.io/specification/2025-11-25/server/resources#resource-contents

Fixes #234.

lib/mcp/resource/contents.rb

@@ -11,7 +11,7 @@ module MCP
       end
 
       def to_h
-        { uri: uri, mime_type: mime_type }.compact
+        { uri: uri, mimeType: mime_type }.compact
       end
     end
 
@@ -37,7 +37,7 @@ module MCP
       end
 
       def to_h
-        super.merge(data: data)
+        super.merge(blob: data)
       end
     end
   end