Fix MerchantDomainService coverage gaps: production bugs + full test coverage

[Copilot]

MerchantDomainService had widespread copy-paste bugs causing merchant aggregate failures to be silently swallowed, a missing return in the balance check path, and large gaps in test coverage across all 15 service methods.

Production Bug Fixes

13 copy-paste guard bugs — every method that fetches both estate and merchant aggregates was checking estateResult.IsFailed twice instead of checking merchantResult.IsFailed on the second check. A failed merchant fetch would fall through with a null/invalid aggregate:

// Before (wrong in 13 methods)
Result<MerchantAggregate> merchantResult = await DomainServiceHelper.GetAggregateOrFailure(...);
if (estateResult.IsFailed)   // ← always false at this point
    return ResultHelpers.CreateFailure(merchantResult);

// After
if (merchantResult.IsFailed)
    return ResultHelpers.CreateFailure(merchantResult);

Missing return in MakeMerchantWithdrawal — balance fetch failure called Result.Invalid(...) without return, so execution continued into JsonConvert.DeserializeObject on a null string:

// Before
if (getBalanceResult.IsFailed) { Result.Invalid("..."); }  // result discarded

// After  
if (getBalanceResult.IsFailed) { return Result.Invalid("..."); }

Test Coverage

• 15 exception-path tests — one per service method covering the catch (Exception ex) handler via ThrowsAsync
• Previously skipped tests re-enabled — RequireCustomMerchantNumber / RequireCustomTerminalNumber tests were skipped pending an OperatorAggregate; added the missing mock and enabled them
• Switch arm coverage — added SettlementSchedule.NotSet and MerchantDepositSource.Automatic tests to cover the _ and Automatic branches
• Fixed 4 existing withdrawal tests — corrected GetLatest<MerchantAggregate> → Get<MerchantAggregate> to match the actual service call; added missing EventStoreContext mock for the NotEnoughFunds path

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update Test Coverage on Merchant Domain service</issue_title>
<issue_description>Each domain service method needs new tests added to cover all cases, include exception and failure cases please.</issue_description>

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

• Fixes Update Test Coverage on Merchant Domain service #914

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[StuartFerguson]

@copilot the following methods still have missing coverage lines all around the failure cases

AddContractToMerchant
AddDeviceToMerchant
AddMerchantAddress
AddMerchantContact
AssignOperatorToMerchant
CreateMerchantUser
MakeMerchantDeposit
MakeMerchantWithdrawal
RemoveContractFromMerchant
RemoveOperatorFromMerchant
SwapMerchantDevice
UpdateMerchant
UpdateMerchantAddress
UpdateMerchantContact

[StuartFerguson]

@copilot still have some missing coverage as mentiond aboce can you review this