TransactionAggregate.ValidateContext rejects missing DeviceIdentifier for non-online transactions #1972

Description

TransactionAggregate.ValidateContext previously rejected a missing DeviceIdentifier for every transaction source.

That is incorrect for non-online transactions. A device identifier is required for TransactionSource.OnlineSale, but non-online sources such as FileImport may not have one.

Expected behavior

• TransactionSource.OnlineSale with a null or empty DeviceIdentifier should return an invalid result.
• TransactionSource.FileImport or TransactionSource.NotSet with a null or empty DeviceIdentifier should be accepted when the other context fields are valid.

Reproduction

var result = aggregate.StartTransaction(
    transactionDateTime,
    transactionNumber,
    TransactionType.Sale,
    transactionReference,
    new TransactionStartContext
    {
        EstateId = estateId,
        MerchantId = merchantId,
        DeviceIdentifier = null,
        TransactionSource = TransactionSource.FileImport
    },
    amount);

Actual behavior before the fix

The result was invalid solely because DeviceIdentifier was missing, regardless of the transaction source.

Fix coverage

Unit tests should explicitly set TransactionSource and cover both the OnlineSale rejection and the non-online success paths.