[client] Optimize Windows DNS performance with domain batching and batch mode (#5264)

* Optimize Windows DNS performance with domain batching and batch mode

Implement two-layer optimization to reduce Windows NRPT registry operations:

1. Domain Batching (host_windows.go):
  - Batch domains per NRPT
  - Reduces NRPT rules by ~97% (e.g., 184 domains: 184 rules → 4 rules)
  - Modified addDNSMatchPolicy() to create batched NRPT entries
  - Added comprehensive tests in host_windows_test.go

2. Batch Mode (server.go):
  - Added BeginBatch/EndBatch methods to defer DNS updates
  - Modified RegisterHandler/DeregisterHandler to skip applyHostConfig in batch mode
  - Protected all applyHostConfig() calls with batch mode checks
  - Updated route manager to wrap route operations with batch calls

* Update tests

* Fix log line

* Fix NRPT rule index to ensure cleanup covers partially created rules

* Ensure NRPT entry count updates even on errors to improve cleanup reliability

* Switch DNS batch mode logging from Info to Debug level

* Fix batch mode to not suppress critical DNS config updates

Batch mode should only defer applyHostConfig() for RegisterHandler/
DeregisterHandler operations. Management updates and upstream nameserver
failures (deactivate/reactivate callbacks) need immediate DNS config
updates regardless of batch mode to ensure timely failover.

Without this fix, if a nameserver goes down during a route update,
the system DNS config won't be updated until EndBatch(), potentially
delaying failover by several seconds.

Or if you prefer a shorter version:

Fix batch mode to allow immediate DNS updates for critical paths

Batch mode now only affects RegisterHandler/DeregisterHandler.
Management updates and nameserver failures always trigger immediate
DNS config updates to ensure timely failover.

* Add DNS batch cancellation to rollback partial changes on errors

Introduces CancelBatch() method to the DNS server interface to handle error
scenarios during batch operations. When route updates fail partway through, the DNS
server can now discard accumulated changes instead of applying partial state. This
prevents leaving the DNS configuration in an inconsistent state when route manager
operations encounter errors.

The changes add error-aware batch handling to prevent partial DNS configuration
updates when route operations fail, which improves system reliability.

client/internal/dns/server.go

@@ -45,6 +45,9 @@ type IosDnsManager interface {
 type Server interface {
 	RegisterHandler(domains domain.List, handler dns.Handler, priority int)
 	DeregisterHandler(domains domain.List, priority int)
+	BeginBatch()
+	EndBatch()
+	CancelBatch()
 	Initialize() error
 	Stop()
 	DnsIP() netip.Addr
@@ -87,6 +90,7 @@ type DefaultServer struct {
 	currentConfigHash  uint64
 	handlerChain       *HandlerChain
 	extraDomains       map[domain.Domain]int
+	batchMode          bool
 
 	mgmtCacheResolver *mgmt.Resolver
 
@@ -234,7 +238,9 @@ func (s *DefaultServer) RegisterHandler(domains domain.List, handler dns.Handler
 		// convert to zone with simple ref counter
 		s.extraDomains[toZone(domain)]++
 	}
-	s.applyHostConfig()
+	if !s.batchMode {
+		s.applyHostConfig()
+	}
 }
 
 func (s *DefaultServer) registerHandler(domains []string, handler dns.Handler, priority int) {
@@ -263,9 +269,41 @@ func (s *DefaultServer) DeregisterHandler(domains domain.List, priority int) {
 			delete(s.extraDomains, zone)
 		}
 	}
+	if !s.batchMode {
+		s.applyHostConfig()
+	}
+}
+
+// BeginBatch starts batch mode for DNS handler registration/deregistration.
+// In batch mode, applyHostConfig() is not called after each handler operation,
+// allowing multiple handlers to be registered/deregistered efficiently.
+// Must be followed by EndBatch() to apply the accumulated changes.
+func (s *DefaultServer) BeginBatch() {
+	s.mux.Lock()
+	defer s.mux.Unlock()
+	log.Debugf("DNS batch mode enabled")
+	s.batchMode = true
+}
+
+// EndBatch ends batch mode and applies all accumulated DNS configuration changes.
+func (s *DefaultServer) EndBatch() {
+	s.mux.Lock()
+	defer s.mux.Unlock()
+	log.Debugf("DNS batch mode disabled, applying accumulated changes")
+	s.batchMode = false
 	s.applyHostConfig()
 }
 
+// CancelBatch cancels batch mode without applying accumulated changes.
+// This is useful when operations fail partway through and you want to
+// discard partial state rather than applying it.
+func (s *DefaultServer) CancelBatch() {
+	s.mux.Lock()
+	defer s.mux.Unlock()
+	log.Debugf("DNS batch mode cancelled, discarding accumulated changes")
+	s.batchMode = false
+}
+
 func (s *DefaultServer) deregisterHandler(domains []string, priority int) {
 	log.Debugf("deregistering handler with priority %d for %v", priority, domains)
 
@@ -523,6 +561,7 @@ func (s *DefaultServer) applyConfiguration(update nbdns.Config) error {
 		s.currentConfig.RouteAll = false
 	}
 
+	// Always apply host config for management updates, regardless of batch mode
 	s.applyHostConfig()
 
 	s.shutdownWg.Add(1)
@@ -887,6 +926,7 @@ func (s *DefaultServer) upstreamCallbacks(
 			}
 		}
 
+		// Always apply host config when nameserver goes down, regardless of batch mode
 		s.applyHostConfig()
 
 		go func() {
@@ -922,6 +962,7 @@ func (s *DefaultServer) upstreamCallbacks(
 			s.registerHandler([]string{nbdns.RootZone}, handler, priority)
 		}
 
+		// Always apply host config when nameserver reactivates, regardless of batch mode
 		s.applyHostConfig()
 
 		s.updateNSState(nsGroup, nil, true)