Balance Intercompany Journal Entry

    Balances an intercompany journal entry by entity and currency.

    This endpoint performs the same balancing logic as the frontend, applying:
    1. Auto-fetching of exchange rates if not provided (requires date field)
    2. Exchange rate roll-forward from native to book to consolidation currencies
    3. Exchange rate plugging to ensure consolidation amounts match across currencies within an entity
    4. Automatic creation of balancing transactions when debits != credits at any currency level

    The endpoint accepts a partial intercompany journal entry with transactions and optionally exchange rates,
    and returns the balanced version with:
    - Auto-fetched exchange rates (if exchange_rates is empty and date is provided)
    - Updated transactions with calculated book and consolidation amounts
    - New balancing transactions added where needed
    - Plugged exchange rates to maintain consolidation consistency

    This is useful for:
    - Validating journal entries before submission
    - Understanding what balancing entries will be created
    - Testing multi-currency and multi-entity scenarios
    - Auto-fetching exchange rates for the journal date