Cover middleware freeze contracts.

[samuel-williams-shopify]

Summary

• cover custom freeze behavior across session, static, content, controller, localization, redirection, and exception middleware
• verify receivers and owned configuration are frozen
• verify repeated calls to freeze return the same object
• add the standard frozen guard to Controller::Rewrite::ExtractPrefixRule

Testing

• affected suites: 123 tests, 336 assertions
• bundle exec rubocop for all changed files
• bundle exec bake decode:index:coverage lib