Protect eBifrost quorum cache and event ownership

The eBifrost quorum cache and confirmation event path could retain caller-owned protobuf pointers. This change snapshots message payloads at the ownership boundary and adds mutation and concurrency regression tests so later caller changes cannot alter cached or emitted data.

Validation: focused eBifrost tests and the mocknet race suite passed locally. This topic is also included in draft !5181 (closed), whose integrated MR pipeline passed. This MR will run its own CI against develop; review and merge coordination should avoid landing the same change twice.