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.

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