fix(tests): skip vernier on Ruby head due to compilation issues

The vernier gem (v1.9.0) fails to load its native extension on Ruby
head (4.1.0+1). Skip loading vernier on Ruby >= 4.1 until the gem
is updated to support the development Ruby version.

This fixes the test failures for Ruby head with Rack 2, 3, and 3.1.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: mjq

sentry-ruby/spec/spec_helper.rb

@@ -10,7 +10,8 @@
 require "rspec/retry"
 require "redis"
 require "stackprof" unless RUBY_PLATFORM == "java"
-require "vernier" unless RUBY_PLATFORM == "java" || RUBY_VERSION < "3.2"
+# XXX: vernier does not currently compile on Ruby head (4.1)
+require "vernier" unless RUBY_PLATFORM == "java" || RUBY_VERSION < "3.2" || RUBY_VERSION >= "4.1"
 
 SimpleCov.start do
   project_name "sentry-ruby"