Add tests for auction format conversion helpers

[prk-Jr]

Description

Format conversion helpers in crates/common/src/auction/formats.rs are untested, leaving banner/video mapping behavior exposed to silent regressions.

Done when

• Add tests for banner format conversion.
• Add tests for video/default-field behavior.
• Add negative tests for invalid format identifiers or malformed inputs.
• Keep coverage focused on conversion semantics used by auction handling.

Affected area

• Ad serving (Equativ)

Additional context

Source: test_gap.md, section Format Conversion.
Parent issue: #396