Support icons parameter

This PR adds support for the optional `icons` parameter in prompt, resource,
resource template, and server info.

The specifications for `icons` are as follows.
https://modelcontextprotocol.io/specification/2025-11-25/schema#icon

Resolves https://github.com/modelcontextprotocol/ruby-sdk/issues/127.

test/mcp/icon_test.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class IconTest < ActiveSupport::TestCase
+    def test_initialization
+      icon = Icon.new(mime_type: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light")
+
+      assert_equal("image/png", icon.mime_type)
+      assert_equal(["48x48", "96x96"], icon.sizes)
+      assert_equal("https://example.com", icon.src)
+      assert_equal("light", icon.theme)
+
+      assert_equal({ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }, icon.to_h)
+    end
+
+    def test_initialization_by_default
+      icon = Icon.new
+
+      assert_nil(icon.mime_type)
+      assert_nil(icon.sizes)
+      assert_nil(icon.src)
+      assert_nil(icon.theme)
+
+      assert_empty(icon.to_h)
+    end
+
+    def test_valid_theme_for_light
+      assert_nothing_raised do
+        Icon.new(theme: "light")
+      end
+    end
+
+    def test_valid_theme_for_dark
+      assert_nothing_raised do
+        Icon.new(theme: "dark")
+      end
+    end
+
+    def test_invalid_theme
+      exception = assert_raises(ArgumentError) do
+        Icon.new(theme: "unexpected")
+      end
+      assert_equal('The value of theme must specify "light" or "dark".', exception.message)
+    end
+  end
+end