Add missing test coverage for MerchantStatementDomainService

[Copilot]

Each method in MerchantStatementDomainService lacked tests for aggregate fetch failures, save failures, and unhandled exceptions — leaving entire error-handling branches uncovered.

Added test cases

• AddTransactionToStatement — MerchantStatementForDateAggregate fetch failure; exception thrown
• AddSettledFeeToStatement — MerchantStatementForDateAggregate fetch failure; exception thrown
• AddDepositToStatement — MerchantStatementForDateAggregate fetch failure; save failure; exception thrown
• AddWithdrawalToStatement — MerchantStatementForDateAggregate fetch failure; save failure; exception thrown
• RecordActivityDateOnMerchantStatement — MerchantStatementAggregate fetch failure; exception thrown
• GenerateStatement — MerchantStatementAggregate fetch failure; exception thrown
• BuildStatement — MerchantStatementAggregate fetch failure; IStatementBuilder.GetStatementHtml failure; exception thrown
• EmailStatement — MerchantStatementAggregate fetch failure; save failure; exception thrown

Each test follows the existing Moq/Shouldly/TestData conventions. Failure-path tests use ReturnsAsync(Result.Failure()) and exception tests use .ThrowsAsync(new Exception("Test exception")), asserting result.IsFailed.ShouldBeTrue() in both cases:

[Fact]
public async Task MerchantStatementDomainService_AddDepositToStatement_ExceptionThrown_ResultIsFailed()
{
    this.AggregateService
        .Setup(a => a.GetLatest<MerchantStatementForDateAggregate>(It.IsAny<Guid>(), It.IsAny<CancellationToken>()))
        .ThrowsAsync(new Exception("Test exception"));

    Result result = await this.DomainService.AddDepositToStatement(
        TestData.Commands.AddDepositToMerchantStatementCommand, CancellationToken.None);

    result.IsFailed.ShouldBeTrue();
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update Test Coverage on Merchant Statement Domain service</issue_title>
<issue_description>Each domain service method needs new tests added to cover all cases</issue_description>

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

• Fixes Update Test Coverage on Merchant Statement Domain service #928

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[StuartFerguson]

@copilot can you add exception cases to the tests for each method

[Copilot]

@copilot can you add exception cases to the tests for each method

Added exception test cases for all 8 methods in commit 9059e68. Each test configures the first mock call to ThrowsAsync(new Exception("Test exception")) and asserts result.IsFailed.ShouldBeTrue(), verifying the catch (Exception ex) block in each method returns Result.Failure.