Add size field to MCP::Resource per MCP specification

## Motivation and Context

The MCP specification defines an optional `size` field on the `Resource`
type, added in spec revision 2025-06-18 and retained in 2025-11-25:
https://modelcontextprotocol.io/specification/2025-11-25/server/resources

> The size of the raw resource content, in bytes (i.e., before base64
> encoding or any tokenization), if known. This can be used by Hosts to
> display file sizes and estimate context window usage.

`MCP::Resource` was missing this field, so servers had no way to declare a
resource's byte size. Without it, hosts cannot show file sizes or estimate
context window usage before reading the resource contents.

This aligns the Ruby SDK with the spec, matching the TypeScript SDK
(modelcontextprotocol/typescript-sdk#1574).

## How Has This Been Tested?

Added tests in `test/mcp/resource_test.rb`:

- `#to_h` omits `size` when nil
- `#to_h` includes `size` when present
- `#to_h` includes `size` when zero (distinguishes an unset size from a
  0-byte resource)

The full unit suite and RuboCop pass locally.

## Breaking Changes

None. `size:` is a new optional keyword argument defaulting to `nil`, and it
is omitted from `#to_h` output when not set, so existing callers are unaffected.

lib/mcp/resource.rb

@@ -5,15 +5,16 @@ require_relative "resource/embedded"
 
 module MCP
   class Resource
-    attr_reader :uri, :name, :title, :description, :icons, :mime_type, :meta
+    attr_reader :uri, :name, :title, :description, :icons, :mime_type, :size, :meta
 
-    def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil, meta: nil)
+    def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil, size: nil, meta: nil)
       @uri = uri
       @name = name
       @title = title
       @description = description
       @icons = icons
       @mime_type = mime_type
+      @size = size
       @meta = meta
     end
 
@@ -25,6 +26,7 @@ module MCP
         description: description,
         icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         mimeType: mime_type,
+        size: size,
         _meta: meta,
       }.compact
     end

test/mcp/resource_test.rb

@@ -49,5 +49,23 @@ module MCP
 
       assert_equal meta, resource.to_h[:_meta]
     end
+
+    test "#to_h omits size when nil" do
+      resource = Resource.new(uri: "file:///test.txt", name: "resource_without_size")
+
+      refute resource.to_h.key?(:size)
+    end
+
+    test "#to_h includes size when present" do
+      resource = Resource.new(uri: "file:///test.txt", name: "resource_with_size", size: 12_345)
+
+      assert_equal 12_345, resource.to_h[:size]
+    end
+
+    test "#to_h includes size when zero" do
+      resource = Resource.new(uri: "file:///empty.txt", name: "empty_resource", size: 0)
+
+      assert_equal 0, resource.to_h[:size]
+    end
   end
 end