Revert to Class.new syntax for empty class definitions

Use Class.new(StandardError) instead of empty class definition
for ReplayIdError.

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

Author: knapo

lib/salesforce_streamer/errors.rb

@@ -1,6 +1,5 @@
 module SalesforceStreamer
-  class ReplayIdError < StandardError
-  end
+  ReplayIdError = Class.new(StandardError)
 
   class MissingCLIFlagError < StandardError
     def initialize(flag)