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 up to 50 domains per NRPT rule (Windows undocumented limit)
  - 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

client/internal/dns/server_export_test.go

@@ -18,7 +18,12 @@ func TestGetServerDns(t *testing.T) {
 		t.Errorf("invalid dns server instance: %s", err)
 	}
 
-	if srvB != srv {
+	mockSrvB, ok := srvB.(*MockServer)
+	if !ok {
+		t.Errorf("returned server is not a MockServer")
+	}
+
+	if mockSrvB != srv {
 		t.Errorf("mismatch dns instances")
 	}
 }