Fix JSON 3 message generation

[samuel-williams-shopify]

Summary

Make the built-in JSON coder’s behavior explicit: incoming object keys are symbolized, while outgoing messages use plain JSON.generate. This prevents parser-only symbolize_names from being passed to JSON 3’s generator.

Protocol::WebSocket::Coder::JSON is now a stateless module and is itself the default coder, removing the redundant JSON::DEFAULT instance. Applications needing different serialization behavior can pass any coder responding to parse and generate.

Migration

Replace uses of Protocol::WebSocket::Coder::JSON.new(...) or JSON::DEFAULT with Protocol::WebSocket::Coder::JSON. Use a custom coder when different JSON options are required.

Validation

• 96 tests pass (161 assertions)
• Direct JSON 3.0.1 reproduction passes
• RuboCop passes
• Documentation coverage remains 100%