Fix webhook registration for topics containing dots

Fixes an issue where webhook topics containing dots (e.g., customer.tags_added,
customer.tags_removed) would fail to register with GraphQL syntax errors.

The problem was an inconsistency in topic name normalization:
- Registration.rb only replaced slashes with underscores
- Registry.rb replaced both slashes and dots with underscores

This caused GraphQL field name errors since dots are invalid in GraphQL field names.

Updated the normalization in Registration.rb to match Registry.rb by replacing
both slashes and dots with underscores.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: lizkenyon

lib/shopify_api/webhooks/registration.rb

@@ -32,7 +32,7 @@ class Registration
           filter: T.nilable(String)).void
       end
       def initialize(topic:, path:, handler: nil, fields: nil, metafield_namespaces: nil, filter: nil)
-        @topic = T.let(topic.gsub("/", "_").upcase, String)
+        @topic = T.let(topic.gsub(%r{/|\.}, "_").upcase, String)
         @path = path
         @handler = handler
         fields_array = fields.is_a?(String) ? fields.split(FIELDS_DELIMITER) : fields