fix: missing validator_count in head sync path

[olegshmuelov]

Summary

• Fix UpdateClusterIfExists not writing validator_count, causing stale
  cluster state when validators are added/removed between finalized and head
• Add clusterUpdates count to SyncClustersToHead debug log for observability
• Update test to verify validator_count is written

Root cause

The head sync path (applyClusterUpdates → UpdateClusterIfExists) was missing
validator_count from both the ClusterRow literal and the SQL UPDATE statement.
This caused IncorrectClusterState failures on Hoodi when a validator was added
between finalized and head blocks.