Extract config_file method from action_class to enable external access

[Copilot]

The nginx_site resource previously defined the config_file method inside the action_class block, making it inaccessible from resource instances. This prevented useful usage patterns like:

site = nginx_site 'service-discovery' do
  conf_dir ::File.join(config.nginx_dir, 'sites-available')
  cookbook 'dsi-consul-servers'
  template 'service-discovery.conf.erb'
  variables(
     ...
  )
  action :create
end

link site.config_file do    # This would fail before this change
  to "../sites-enabled/#{site.name}"
end

This PR moves the config_file method from inside the action_class block to the main resource body, making it accessible on resource instances while preserving all existing functionality.

Changes made:

• Moved config_file method definition outside of action_class
• Updated method implementation to use conf_dir and name directly instead of new_resource.conf_dir and new_resource.name
• Added test case to verify external accessibility works correctly
• All existing internal usage continues to work unchanged

Result:
Users can now access the configuration file path externally:

site = nginx_site 'my-site' do
  conf_dir '/etc/nginx/sites-available'
  action :create
end

puts site.config_file  # Returns: /etc/nginx/sites-available/my-site.conf

This is a minimal, backward-compatible change that enables the requested functionality without breaking any existing behavior.

Fixes #620.

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[damacus]

Extract this to the helpers.rb