Omit icons from responses when empty or nil

Previously, the `icons` parameter was always included in responses (as an empty array)
even when not configured. This differs from the response representation in MCP Ruby SDK 0.4.0.

Since `icons` is optional, it should be omitted from responses when unused to reduce
unnecessary context window usage.
https://modelcontextprotocol.io/specification/2025-11-25/schema#implementation

This fix applies to `Tool#to_h`, `Prompt#to_h`, `Resource#to_h`, `ResourceTemplate#to_h`,
and `Server#server_info`.

It doesn't seem critical, but if there are no other issues, it would be good to cut 0.5.1 bugfix release soon.

test/mcp/resource_test.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class ResourceTest < ActiveSupport::TestCase
+    test "#to_h does not have `:icons` key when icons is empty" do
+      resource = Resource.new(
+        uri: "file:///test.txt",
+        name: "resource_without_icons",
+        description: "a resource without icons",
+      )
+
+      refute resource.to_h.key?(:icons)
+    end
+
+    test "#to_h does not have `:icons` key when icons is nil" do
+      resource = Resource.new(
+        uri: "file:///test.txt",
+        name: "resource_without_icons",
+        description: "a resource without icons",
+        icons: nil,
+      )
+
+      refute resource.to_h.key?(:icons)
+    end
+
+    test "#to_h includes icons when present" do
+      resource = Resource.new(
+        uri: "file:///test.txt",
+        name: "resource_with_icons",
+        description: "a resource with icons",
+        icons: [Icon.new(mime_type: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light")],
+      )
+      expected_icons = [{ mimeType: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light" }]
+
+      assert_equal expected_icons, resource.to_h[:icons]
+    end
+  end
+end