Consolidate duplicate callback serde rename tests

[prk-Jr]

Description

crates/common/src/models.rs currently uses overlapping callback deserialization tests to verify the same serde rename behavior. This is redundant and makes the test surface noisier than necessary.

Done when

• Consolidate the duplicate callback rename tests into one focused test.
• Preserve the existing type -> callback_type serde assertion.
• Keep the resulting test intent clearer than the current duplicated coverage.

Affected area

• Core (synthetic IDs, cookies, GDPR)

Additional context

Source: test_gap.md, section Duplicate Callback Deserialization Tests.
Parent issue: #396