Backport to testnet_conway: Arc deeper through certificate/block paths + Arc newtype (#6147 + #6330)

[ndr-ds]

Motivation

Supersedes #6249 (which had accumulated merge conflicts). Backports #6147 and #6330 to testnet_conway in a single commit on a rebased branch.

Proposal

Cherry-pick of #6147 rebased on current testnet_conway tip, with conflict resolution, plus the linera_cache::Arc<T> newtype from #6330.

Conflict resolution notes:

• linera-storage/src/lib.rs: took testnet_conway's removals (shared_committees, get_or_load_committee_by_hash, committee_for_epoch, test module); adjusted get_or_load_committee return type to std::sync::Arc<Committee> (Committee is not managed by value cache)
• linera-storage/src/db_storage.rs: took testnet_conway's structure throughout, then applied newtype return-type changes
• linera-core/src/client/chain_client/mod.rs: kept testnet_conway's download_certificates_for_events restructuring and BlobsNotFound handling; adapted blob/certificate boundary conversions (into_std(), as_std().clone())
• linera-service/src/proxy/grpc.rs: kept testnet_conway's if all_found guard; used (&*certificate).into() to avoid .as_ref() ambiguity on CacheArc

Test Plan

CI — cargo check --workspace --exclude linera-web and cargo clippy clean locally before pushing.

Release Plan

• Nothing to do / These changes follow the usual release cycle.

Links

• Push Arc deeper through certificate and block paths #6147 — original main PR (merged)
• Add linera_cache::Arc<T> newtype enforcing one-allocation-per-content invariant #6330 — the newtype follow-up on main
• Backport to testnet_conway: Push Arc deeper through certificate and block paths (#6147) #6249 — superseded backport PR
• reviewer checklist