Fix NoMethodError for us_postage_type on Letter::Batch instances

[Copilot]

The Letter::BatchesController#validate_postage_types method was failing with a NoMethodError: undefined method 'us_postage_type' for an instance of Letter::Batch when trying to validate postage types during batch updates.

Root Cause

The controller's validate_postage_types method attempts to access @batch.us_postage_type and @batch.intl_postage_type:

def validate_postage_types
  return unless @batch.letter_return_address&.us?

  if @batch.us_postage_type.present? && !%w[stamps indicia].include?(@batch.us_postage_type)
    @batch.errors.add(:us_postage_type, "must be either 'stamps' or 'indicia'")
  end
  # ...
end

However, these attributes don't exist on the Letter::Batch model - they are form parameters used for processing but not persisted to the database.

Solution

Added us_postage_type and intl_postage_type as virtual attributes using attr_accessor in the Letter::Batch model:

attr_accessor :template, :template_cycle, :us_postage_type, :intl_postage_type

This allows the form parameters to be set on the batch instance during updates and accessed during validation, without requiring database columns. The approach follows existing patterns in the codebase (e.g., from_letter attribute in ReturnAddress model).

Testing

Verified the fix handles all scenarios correctly:

• Valid postage types ("stamps", "indicia") pass validation
• Invalid postage types trigger appropriate validation errors
• Empty/nil values are handled properly
• The controller update flow works as expected

Fixes #156.

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