IAV-985 : refactor free trial debit logic to cascade balance usage

• Change: free trial debit now uses available balance instead of throwing exception
• When free trial balance is insufficient, remaining amount cascades to other balance types
• Prerequisite: if free trial balance is already 0 or negative, throws INSUFFICIENT_FREE_TRIAL_BALANCE error
• Benefit: Enables flexible credit consumption (free trial → main → extra → referral)
• Updated error codes and constants for insufficient free trial balance

Closes IAV-985