[TransactionMobile] Add exception-path coverage for sale transaction HTTP failures #658

Background

TransactionProcessor#1840 shows a Sentry failure on the sale transaction request path, with the stack landing in LoggingHttpMessageHandler.SendCoreAsync.

The sale flows in this repo all go through TransactionProcessor.Mobile.BusinessLogic.Services.TransactionService.SendTransactionRequest(...) before posting to api/saletransactions, but the current tests only cover successful responses and non-2xx HTTP statuses.

Scope

Add test coverage for the shared api/saletransactions path when the HTTP client pipeline throws during SendAsync or message-handler execution.

Cover the following flows:

• PerformMobileTopup
• PerformVoucherIssue
• PerformBillPaymentGetAccount
• PerformBillPaymentMakePayment
• PerformBillPaymentGetMeter

Expected behavior

• The service returns a failed result instead of crashing the caller.
• The failure path is exercised when the handler throws, not just when the response status is non-success.
• Any logging or exception wrapping remains observable and covered by test.

Acceptance criteria

• A test fails if the HTTP pipeline throws before a response is returned.
• Success and non-2xx coverage remains green for the sale transaction path.
• The issue is traceable back to TransactionProcessor#1840.

Notes

If the exception is swallowed by a shared HTTP wrapper, place the test at the lowest layer where the exception is actually thrown.