Add GetCrashedReplicaMessages and SetReplica to IMessageStore

[stidsborg]

Summary

Adds two methods to IMessageStore to support re-distributing messages stranded by crashed replicas:

Task<List<Tuple<StoredId, long>>> GetCrashedReplicaMessages(IEnumerable<ReplicaId> liveReplicas);
Task SetReplica(IEnumerable<long> positions, ReplicaId newReplica, ReplicaId expectedReplica);

• GetCrashedReplicaMessages returns the (StoredId, position) identifiers of undelivered messages whose owning replica is not in the supplied live-replica set — i.e. messages stranded by a replica that is no longer alive. It returns just the identifiers (not full StoredMessage content), since the caller only needs them to hand off to SetReplica.
• SetReplica bulk-reassigns the messages at the given positions to newReplica, guarded by expectedReplica (optimistic concurrency) so a concurrent takeover/delivery is not clobbered. position is the global identity PK in every store, so keying on positions alone is unambiguous.

Changes

• Interface (Core/.../Messaging/IMessageStore.cs)
• Implementations: InMemoryFunctionStore, PostgreSqlMessageStore, MariaDbMessageStore, SqlServerMessageStore (+ each store's SqlGenerator). Each GetCrashedReplicaMessages selects only id, position via a lightweight ReadStoredIdAndPositions reader.
• Tests added to the shared MessageStoreTests template (run against all four stores):
  • CrashedReplicaMessagesAreFetched — only messages owned by crashed replicas are returned.
  • MessageReplicaCanBeReassigned — positions owned by the expected replica are reassigned; one owned by a different replica is left untouched (verifies the expectedReplica guard).

Testing

GetCrashedReplicaMessages / SetReplica tests pass on all stores:

• In-memory: 2/2
• PostgreSQL: 2/2
• SqlServer: 2/2
• MariaDB: 2/2