Write unit tests for payment_service`

[Nemenwq]

Dependencies: Implemented service logic (at minimum Issues #30–#32 and whatever else exists in payment_service).

Context:
Ensures payment business logic remains stable.

Scope:

• In scope: Unit tests for payment creation, status transitions, confirmation/expiry edge cases, webhook event emission logic, and error mapping.
• Out of scope: Full multi-service e2e (Issue Write e2e tests for payment creation and confirmation flow` #50) and browser tests.

Implementation Notes:

• Prefer deterministic tests: mock Soroban client (Issue Implement Soroban client service` #55), HTTP calls, and time.
• Include negative tests (invalid inputs, unauthorized access, race conditions).

Acceptance Criteria:

• Coverage > 80%.
• Happy + error paths covered.
• Dependencies mocked.

Files: backend/payment_service/src/*.spec.ts