Add integration tests for auction endpoint request handling

[prk-Jr]

Description

handle_auction in crates/common/src/auction/endpoints.rs has no end-to-end verification for request parsing, timeout behavior, or response shaping.

Done when

• Add integration coverage for a valid auction request returning a success response.
• Add coverage for invalid request payload returning 400.
• Add coverage for timeout or orchestrator failure returning the expected gateway timeout/error response.
• Assert the response contract used by callers stays stable.

Affected area

• Ad serving (Equativ)

Additional context

Source: test_gap.md, section Auction Handler.
Parent issue: #396