Test README instrumentation callback example

Author: sambostock

README.md

@@ -231,6 +231,7 @@ instrumentation_callback = ->(data) { ... }
 ```
 
 **Example:**
+<!-- SNIPPET ID: instrumentation_callback -->
 ```ruby
 config.instrumentation_callback = ->(data) {
   puts "Instrumentation: #{data.inspect}"

test/fixtures/files/code_snippet_wrappers/readme/instrumentation_callback.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+require "mcp"
+
+MCP.configure do |config|
+  eval(File.read("code_snippet.rb"), binding) # rubocop:disable Security/Eval -- We need to run the snippet in this context to pass the config
+
+  config.instrumentation_callback.call({ example: "data" })
+end

test/integration/readme_code_snippets_test.rb

@@ -126,6 +126,12 @@ class ReadmeCodeSnippetsTest < ActiveSupport::TestCase
     )
   end
 
+  test "Instrumentation Callback example works exactly as documented in README" do
+    assert_equal(<<~STDOUT, run_code_snippet("instrumentation_callback"))
+      Instrumentation: {example: "data"}
+    STDOUT
+  end
+
   private
 
   def assert_json_lines(expected, actual, message = "Expected the given JSON lines")