Extract MCP::Annotations into a dedicated file

This PR moves `MCP::Annotations` from lib/mcp.rb to lib/mcp/annotations.rb for better organization,
following the pattern used for other classes such as `Tool::Annotations`.

This PR also adds the missing `last_modified` attribute to `MCP::Annotations`:
https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-11-25/schema.ts#L1701-L1736

It also adds a `to_h` method for serialization and includes comprehensive test coverage.

NOTE: This code is unused within the ruby-sdk repository, but it is kept for compatibility, as it may be used by users.

lib/mcp.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative "json_rpc_handler"
+require_relative "mcp/annotations"
 require_relative "mcp/configuration"
 require_relative "mcp/content"
 require_relative "mcp/icon"
@@ -39,13 +40,4 @@ module MCP
       @configuration ||= Configuration.new
     end
   end
-
-  class Annotations
-    attr_reader :audience, :priority
-
-    def initialize(audience: nil, priority: nil)
-      @audience = audience
-      @priority = priority
-    end
-  end
 end

lib/mcp/annotations.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module MCP
+  class Annotations
+    attr_reader :audience, :priority, :last_modified
+
+    def initialize(audience: nil, priority: nil, last_modified: nil)
+      @audience = audience
+      @priority = priority
+      @last_modified = last_modified
+    end
+
+    def to_h
+      {
+        audience: audience,
+        priority: priority,
+        lastModified: last_modified,
+      }.compact
+    end
+  end
+end