Add tests for TrustedServerError HTTP status mapping

[prk-Jr]

Description

TrustedServerError::status_code() in crates/common/src/error.rs has no direct tests asserting the HTTP status mapping for each error variant. A regression here would silently change client behavior and monitoring semantics.

Done when

• Add deterministic tests covering each public error variant to status code mapping.
• Assert representative server-side and client-side errors map to the expected HTTP responses.
• Keep the scope limited to mapping coverage, not unrelated error refactors.

Affected area

• Core (synthetic IDs, cookies, GDPR)

Additional context

Source: test_gap.md, section HTTP Status Code Mapping Not Validated.
Parent issue: #396
Related: #437 covers leaked error context; this issue is specifically for missing mapping tests.