Add annotations field to MCP::Resource and MCP::ResourceTemplate per MCP specification

## Motivation and Context

The MCP specification defines an optional `annotations` field on both the
`Resource` and `ResourceTemplate` types, but the Ruby SDK is the only official
SDK that does not wire it up — TypeScript, Python, and PHP all implement it.
https://modelcontextprotocol.io/specification/2025-11-25/server/resources#annotations

The `MCP::Annotations` class already exists; this commit connects it. The field
accepts an `MCP::Annotations` instance rather than a raw hash, for consistency
with other typed fields and to get `lastModified` key normalization for free.

## How Has This Been Tested?

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

- `#to_h` omits `annotations` when nil
- `#to_h` includes `annotations` when present (also asserting that
  `last_modified` is serialized as `lastModified`)

The full unit suite and RuboCop pass locally.

## Breaking Changes

None. `annotations:` is a new optional keyword argument defaulting to `nil`,
and it is omitted from `#to_h` output when not set.

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, :size, :meta
+    attr_reader :uri, :name, :title, :description, :icons, :mime_type, :annotations, :size, :meta
 
-    def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil, size: nil, meta: nil)
+    def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil, annotations: nil, size: nil, meta: nil)
       @uri = uri
       @name = name
       @title = title
       @description = description
       @icons = icons
       @mime_type = mime_type
+      @annotations = annotations
       @size = size
       @meta = meta
     end
@@ -26,6 +27,7 @@ module MCP
         description: description,
         icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         mimeType: mime_type,
+        annotations: annotations&.to_h,
         size: size,
         _meta: meta,
       }.compact

lib/mcp/resource_template.rb

@@ -2,15 +2,16 @@
 
 module MCP
   class ResourceTemplate
-    attr_reader :uri_template, :name, :title, :description, :icons, :mime_type, :meta
+    attr_reader :uri_template, :name, :title, :description, :icons, :mime_type, :annotations, :meta
 
-    def initialize(uri_template:, name:, title: nil, description: nil, icons: [], mime_type: nil, meta: nil)
+    def initialize(uri_template:, name:, title: nil, description: nil, icons: [], mime_type: nil, annotations: nil, meta: nil)
       @uri_template = uri_template
       @name = name
       @title = title
       @description = description
       @icons = icons
       @mime_type = mime_type
+      @annotations = annotations
       @meta = meta
     end
 
@@ -22,6 +23,7 @@ module MCP
         description: description,
         icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         mimeType: mime_type,
+        annotations: annotations&.to_h,
         _meta: meta,
       }.compact
     end

test/mcp/resource_template_test.rb

@@ -53,5 +53,23 @@ module MCP
 
       assert_equal meta, resource_template.to_h[:_meta]
     end
+
+    test "#to_h omits annotations when nil" do
+      resource_template = ResourceTemplate.new(uri_template: "file:///{path}", name: "template_without_annotations")
+
+      refute resource_template.to_h.key?(:annotations)
+    end
+
+    test "#to_h includes annotations when present" do
+      annotations = Annotations.new(audience: ["user"], priority: 0.8, last_modified: "2025-01-12T15:00:58Z")
+      resource_template = ResourceTemplate.new(
+        uri_template: "file:///{path}",
+        name: "template_with_annotations",
+        annotations: annotations,
+      )
+
+      expected = { audience: ["user"], priority: 0.8, lastModified: "2025-01-12T15:00:58Z" }
+      assert_equal expected, resource_template.to_h[:annotations]
+    end
   end
 end

test/mcp/resource_test.rb

@@ -67,5 +67,19 @@ module MCP
 
       assert_equal 0, resource.to_h[:size]
     end
+
+    test "#to_h omits annotations when nil" do
+      resource = Resource.new(uri: "file:///test.txt", name: "resource_without_annotations")
+
+      refute resource.to_h.key?(:annotations)
+    end
+
+    test "#to_h includes annotations when present" do
+      annotations = Annotations.new(audience: ["user"], priority: 0.8, last_modified: "2025-01-12T15:00:58Z")
+      resource = Resource.new(uri: "file:///test.txt", name: "resource_with_annotations", annotations: annotations)
+
+      expected = { audience: ["user"], priority: 0.8, lastModified: "2025-01-12T15:00:58Z" }
+      assert_equal expected, resource.to_h[:annotations]
+    end
   end
 end