Add missing test coverage for VoucherDomainService failure paths

[Copilot]

VoucherDomainService lacked test coverage for aggregate fetch failures and save failures in both IssueVoucher and RedeemVoucher.

New Tests

IssueVoucher

• VoucherDomainService_IssueVoucher_GetVoucherFailed_ErrorThrown — GetLatest<VoucherAggregate> returns Result.Failure() after estate validation passes
• VoucherDomainService_IssueVoucher_SaveFailed_ErrorThrown — Save returns Result.Failure after successful generate/barcode/issue steps

RedeemVoucher

• VoucherDomainService_RedeemVoucher_GetVoucherAggregateFailed_ErrorThrown — GetLatest<VoucherAggregate> returns Result.Failure() after voucher projection is found in DB and estate validation passes
• VoucherDomainService_RedeemVoucher_SaveFailed_ErrorThrown — Save returns Result.Failure after successful redeem step

Each test asserts result.IsFailed.ShouldBeTrue(), consistent with the pattern used across other domain service test files (FloatDomainServiceTests, EstateDomainServiceTests).

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• f.feedz.io
  • Triggering command: /usr/bin/dotnet dotnet test TransactionProcessor.BusinessLogic.Tests/TransactionProcessor.BusinessLogic.Tests.csproj --filter FullyQualifiedName~VoucherDomainService --no-build (dns block)
  • Triggering command: /usr/bin/dotnet dotnet build TransactionProcessor.BusinessLogic.Tests/TransactionProcessor.BusinessLogic.Tests.csproj (dns block)
  • Triggering command: /usr/bin/dotnet dotnet restore TransactionProcessor.BusinessLogic.Tests/TransactionProcessor.BusinessLogic.Tests.csproj --ignore-failed-sources (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update Test Coverage on Voucher Domain Service</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update Test Coverage on Voucher Domain Service #931

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