Add @WebMvcTest slice test for FlashSaleController

[piush365]

Background

FlashSaleController is only covered today by the full-stack FlashSaleIntegrationTest. A @WebMvcTest slice test is much faster (< 1 s vs. > 30 s) and pinpoints controller-layer bugs — validation, HTTP status mapping, response body shape — without needing Redis, Kafka, or a database.

What to Test

@WebMvcTest(FlashSaleController.class)
class FlashSaleControllerTest {

    @MockBean FlashSaleService flashSaleService;

    @Test void buy_returns202_whenAccepted() { ... }
    @Test void buy_returns410_whenSoldOut() { ... }
    @Test void buy_returns429_whenRateLimited() { ... }
    @Test void buy_returns409_whenConcurrentBlock() { ... }
    @Test void buy_returns503_whenCircuitOpen() { ... }
    @Test void buy_returns500_whenError() { ... }
    @Test void buy_returns400_withBlankUserId() { ... }
    @Test void buy_returns401_withoutApiKey() { ... }
    @Test void responseBody_containsCorrelationId() { ... }
}

Notes

• Mock FlashSaleService — no infrastructure needed.
• ApiKeyAuthFilter must still be wired in (use the test API key from TestPropertySource).
• Assert response JSON shape using jsonPath assertions.
• This is a good first contribution — the controller logic is straightforward and well-documented.

Acceptance Criteria

• All 9 test methods above are implemented and pass
• Test runs in under 5 seconds
• No real Redis, Kafka, or database connections made