Extract trace providers and cover static serving.

[samuel-williams-shopify]

Summary

• move Utopia content and static instrumentation into opt-in traces/provider/utopia providers
• validate emitted trace names and attributes with the traces test backend
• cover callable cache-control policies, strong If-Range matching, and MIME expansion errors

Testing

• bundle exec sus test/traces/provider/utopia/content/middleware.rb test/traces/provider/utopia/static/middleware.rb test/utopia/static.rb
• bundle exec rubocop config/sus.rb lib/utopia/content/middleware.rb lib/utopia/static/middleware.rb lib/traces/provider/utopia.rb lib/traces/provider/utopia/content/middleware.rb lib/traces/provider/utopia/static/middleware.rb test/traces/provider/utopia/content/middleware.rb test/traces/provider/utopia/static/middleware.rb test/utopia/static.rb
• bundle exec bake decode:index:coverage lib