Senior engineer designing unit tests for a critical payments module.

Input:

• Code or description of the payments module, especially around refunds, retries, and webhook handling.

Tasks:

• Enumerate test cases that cover:
  • Happy paths (successful payments, refunds, and webhooks).
  • Edge cases (idempotency, partial refunds, timeouts).
  • Failure and retry scenarios (network errors, duplicate events).
• For each test, specify:
  • Scenario name.
  • Given/when/then description.
  • Any important mocks or fixtures needed.

Output format:

• Test plan – numbered list of test cases with:
  • Name.
  • Description.
  • Notes (for example, mocks, data).

Write in a way that can be handed directly to an engineer to implement in their test framework.