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.

Author: koic

lib/mcp/resource/embedded.rb

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

test/mcp/resource/embedded_test.rb

@@ -0,0 +1,107 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Resource
+    class EmbeddedTest < ActiveSupport::TestCase
+      test "initializes with resource" do
+        resource = Resource.new(
+          uri: "file:///test.txt",
+          name: "test_resource",
+          description: "a test resource",
+        )
+        embedded = Resource::Embedded.new(resource: resource)
+
+        assert_equal resource, embedded.resource
+      end
+
+      test "initializes with resource and annotations" do
+        resource = Resource.new(
+          uri: "file:///test.txt",
+          name: "test_resource",
+          description: "a test resource",
+        )
+        annotations = { audience: ["user"], priority: 1.0 }
+        embedded = Resource::Embedded.new(resource: resource, annotations: annotations)
+
+        assert_equal resource, embedded.resource
+        assert_equal annotations, embedded.annotations
+      end
+
+      test "initializes with annotations as nil when not provided" do
+        resource = Resource.new(
+          uri: "file:///test.txt",
+          name: "test_resource",
+          description: "a test resource",
+        )
+        embedded = Resource::Embedded.new(resource: resource)
+
+        assert_nil embedded.annotations
+      end
+
+      test "#to_h returns hash with resource data when annotations is nil" do
+        resource = Resource.new(
+          uri: "file:///test.txt",
+          name: "test_resource",
+          description: "a test resource",
+        )
+        embedded = Resource::Embedded.new(resource: resource)
+
+        expected = {
+          resource: {
+            uri: "file:///test.txt",
+            name: "test_resource",
+            description: "a test resource",
+          },
+        }
+
+        assert_equal expected, embedded.to_h
+      end
+
+      test "#to_h returns hash with resource and annotations when both are present" do
+        resource = Resource.new(
+          uri: "file:///test.txt",
+          name: "test_resource",
+          description: "a test resource",
+        )
+        annotations = { audience: ["user"], priority: 1.0 }
+        embedded = Resource::Embedded.new(resource: resource, annotations: annotations)
+
+        expected = {
+          resource: {
+            uri: "file:///test.txt",
+            name: "test_resource",
+            description: "a test resource",
+          },
+          annotations: { audience: ["user"], priority: 1.0 },
+        }
+
+        assert_equal expected, embedded.to_h
+      end
+
+      test "#to_h handles resource with all optional fields" do
+        resource = Resource.new(
+          uri: "file:///test.txt",
+          name: "test_resource",
+          title: "Test Resource",
+          description: "a test resource",
+          mime_type: "text/plain",
+        )
+        embedded = Resource::Embedded.new(resource: resource)
+
+        expected = {
+          resource: {
+            uri: "file:///test.txt",
+            name: "test_resource",
+            title: "Test Resource",
+            description: "a test resource",
+            mimeType: "text/plain",
+          },
+        }
+
+        assert_equal expected, embedded.to_h
+      end
+    end
+  end
+end