Refactor PaymentService for testability, add interfaces + factory, and add unit tests

[f-regal]

Description

• Refactor payment logic to decouple storage from business rules by introducing IAccountDataStore and IAccountDataStoreFactory and a default AccountDataStoreFactory.
• Make PaymentService depend on the factory (constructor injection) while preserving the MakePayment method signature and backwards compatibility via a default constructor.
• Implement existing concrete stores to satisfy the new interface.
• Add unit test project dependencies (xUnit, Microsoft.NET.Test.Sdk, test runner) and CopyLocalLockFileAssemblies to ensure runtime dependencies are available for testhost.
• Add unit tests (PaymentServiceTests) exercising BACS, FasterPayments and CHAPS rules; include an ExampleTests sample.
• Small build/test/runtime fixes to avoid testhost deps issues encountered earlier.

What changed (files)

• Modified
  • PaymentService.cs
  • AccountDataStore.cs
  • BackupAccountDataStore.cs
  • ClearBank.DeveloperTest.Tests.csproj
• Added
  • IAccountDataStore.cs
  • IAccountDataStoreFactory.cs
  • AccountDataStoreFactory.cs
  • PaymentServiceTests.cs

Motivation / rationale

• Improve adherence to SOLID (Single Responsibility, Dependency Inversion) and make payment logic easily unit-testable without hitting a real datastore.
• Preserve original public API (MakePayment signature) as required.
• Fix test runtime failures caused by missing transitive runtime assemblies in test output by ensuring test SDK and CopyLocalLockFileAssemblies are configured.

Behavioral notes and compatibility

• Business logic behavior is preserved; existing callers of new PaymentService() continue to work with default factory that reads configuration.
• Tests are pure unit tests using fakes, no external I/O.

How to run and verify

• Ensure .NET 8 SDK installed.
• From repo root:

dotnet restore
dotnet build
dotnet test

• Expected: build succeeds and tests run (unit tests pass).