test: add comprehensive PaymentService routing and lifecycle unit tests

[teesofttech]

Why

Most current tests cover shared helpers and PeachPayments. Core payment orchestration has little direct coverage despite handling routing, transaction persistence, verification updates, refunds, and error wrapping.

Scope

• Test create, automatic/explicit routing, no-provider, unsupported currency, and provider failure.
• Test transaction persistence on success and no persistence on failure.
• Test verification via database/reference and status transitions.
• Test full/partial/cumulative refund rules.
• Test duplicate gateway registrations and malformed responses.

Acceptance criteria

• PaymentService branches are covered with deterministic fakes.
• Tests assert repository side effects and no-side-effect failures.
• Routing cases cover every currency group and DefaultGateway behavior.
• Concurrency and idempotency invariants have dedicated tests.
• Coverage report exposes core service line/branch coverage.