Fix Resource::Embedded not setting @resource in initialize

The `initialize` method was missing the assignment of `@resource`,
causing the resource method to always return `nil` and `to_h` to return
an incorrect hash with an empty resource value.

Added comprehensive tests to verify the fix and prevent regression.

lib/mcp/resource/embedded.rb

@@ -6,6 +6,7 @@ module MCP
       attr_reader :resource, :annotations
 
       def initialize(resource:, annotations: nil)
+        @resource = resource
         @annotations = annotations
       end