feat(services): reduce list fallback stat for local kv backends

[Xuanwo]

Which issue does this PR close?

Part of #7062.

Rationale for this change

CompleteLayer now guarantees content_length for file entries returned by list, but backends that don't populate size in lister metadata still trigger per-entry fallback stat calls.

For local KV-like backends where size is already available at list time, we can fill content_length directly in service listers and avoid unnecessary fallback stat requests.

What changes are included in this PR?

• compfs: populate file content_length from DirEntry::metadata().len() in lister.
• dashmap: lister now reuses cached Metadata instead of reconstructing metadata from key suffix.
• mini_moka: lister now reuses cached Metadata.
• moka: lister now reuses cached Metadata.
• rocksdb: core list now returns (key, value_len) and lister sets file content_length directly.
• sled: core list now returns (key, value_len) and lister sets file content_length directly.

Are there any user-facing changes?

Yes.

For compfs, dashmap, mini-moka, moka, rocksdb, and sled, file entries returned by list now include content_length directly from lister metadata, which reduces extra fallback stat requests.

AI Usage Statement

This PR was developed with Codex (GPT-5) assistance.