fix(core): trim metadata debug output

[Xuanwo]

Which issue does this PR close?

N/A.

Rationale for this change

Metadata currently relies on the derived Debug implementation, which prints every optional field and default boolean state. That makes Entry debug output much noisier than the actual signal, especially during list/debug flows.

What changes are included in this PR?

This change replaces the derived Debug implementation on Metadata with a manual formatter that always prints mode and only prints meaningful fields. Present optional fields are rendered as their inner values instead of Some(...), and unit tests were added to lock in the new output.

Are there any user-facing changes?

Users inspecting Debug output for Metadata and Entry will now see shorter, easier-to-scan output. There is no API behavior change.

AI Usage Statement

Assisted by OpenAI GPT-5.