FINERACT-2656: Fix NPE disbursing a loan with an account-transfer charge and no linked savings account

[oluexpert99]

Description

• Guard retriveLoanLinkedAssociation(...) for null in disburseLoan's account-transfer
  disbursement-charge loop, the down-payment auto-transfer block, and bulkLoanDisbursal's
  charge loop, throwing LinkedAccountRequiredException (as the disburse-to-savings path
  already does) instead of NullPointerException / HTTP 500.
• Guard both TransferFeeChargeForLoansTasklet call sites so the scheduled job records the
  error and skips the misconfigured loan instead of aborting the batch.
• Add LoanWritePlatformServiceJpaRepositoryImplTest coverage asserting the exception is
  raised and no account transfer is attempted when the loan has no linked savings account.

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