Fix READY event handler initialization

[ncoop57]

Fixes an issue where the READY event handler wasn't properly registered during GatewayClient
initialization.

• Move READY handler registration to init to ensure it's always available
• Simplify on() method since handlers dict is now guaranteed to exist
• Remove redundant gc.on('READY', gc.on_rdy) call that was executed at module level

This ensures the gateway can properly capture session_id and resume_url on initial connection.