hscontrol/state,db: preserve node expiry on MapRequest updates

Fixes a regression introduced in v0.27.0 where node expiry times were
being reset to zero when tailscaled restarts and sends a MapRequest.

The issue was caused by using GORM's Save() method in persistNodeToDB(),
which overwrites ALL fields including zero values. When a MapRequest
updates a node (without including expiry information), Save() would
overwrite the database expiry field with a zero value.

Changed to use Updates() which only updates non-zero values, preserving
existing database values when struct pointer fields are nil.

In BackfillNodeIPs, we need to explicitly update IPv4/IPv6 fields even
when nil (to remove IPs), so we use Select() to specify those fields.

Added regression test that validates expiry is preserved after MapRequest.

Fixes #2862

hscontrol/state/state.go

@@ -386,7 +386,11 @@ func (s *State) persistNodeToDB(node types.NodeView) (types.NodeView, change.Cha
 
 	nodePtr := node.AsStruct()
 
-	if err := s.db.DB.Save(nodePtr).Error; err != nil {
+	// Use Omit("expiry") to prevent overwriting expiry during MapRequest updates.
+	// Expiry should only be updated through explicit SetNodeExpiry calls or re-registration.
+	// See: https://github.com/juanfont/headscale/issues/2862
+	err := s.db.DB.Omit("expiry").Updates(nodePtr).Error
+	if err != nil {
 		return types.NodeView{}, change.EmptySet, fmt.Errorf("saving node: %w", err)
 	}