Stabilize the node agent runtime loop.

Keep the NAS-side runtime bounded to the configured export path,
make WebDAV and registration behavior env-driven, and add runtime
coverage so the first storage loop can be verified locally.

Generated with [Devin](https://cli.devin.ai/docs)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/node-agent/internal/nodeagent/app_test.go

@@ -0,0 +1,95 @@
+package nodeagent
+
+import (
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestNewRejectsMissingExportDirectory(t *testing.T) {
+	t.Parallel()
+
+	exportPath := filepath.Join(t.TempDir(), "missing-export")
+
+	_, err := New(Config{
+		ExportPath:    exportPath,
+		ListenAddress: defaultListenAddress(defaultPort),
+	}, log.New(io.Discard, "", 0))
+	if err == nil {
+		t.Fatal("expected missing export directory to fail")
+	}
+
+	if !strings.Contains(err.Error(), "does not exist") {
+		t.Fatalf("error = %q, want missing-directory message", err.Error())
+	}
+}
+
+func TestNewRejectsFileExportPath(t *testing.T) {
+	t.Parallel()
+
+	exportPath := filepath.Join(t.TempDir(), "export.txt")
+	if err := os.WriteFile(exportPath, []byte("not a directory"), 0o644); err != nil {
+		t.Fatalf("write export file: %v", err)
+	}
+
+	_, err := New(Config{
+		ExportPath:    exportPath,
+		ListenAddress: defaultListenAddress(defaultPort),
+	}, log.New(io.Discard, "", 0))
+	if err == nil {
+		t.Fatal("expected file export path to fail")
+	}
+
+	if !strings.Contains(err.Error(), "is not a directory") {
+		t.Fatalf("error = %q, want not-a-directory message", err.Error())
+	}
+}
+
+func TestNewRejectsInvalidListenAddress(t *testing.T) {
+	t.Parallel()
+
+	_, err := New(Config{
+		ExportPath:    t.TempDir(),
+		ListenAddress: "localhost",
+	}, log.New(io.Discard, "", 0))
+	if err == nil {
+		t.Fatal("expected invalid listen address to fail")
+	}
+
+	if !strings.Contains(err.Error(), listenAddressEnvKey) {
+		t.Fatalf("error = %q, want %q", err.Error(), listenAddressEnvKey)
+	}
+}
+
+func TestNewAcceptsLoopbackListenAddressByDefault(t *testing.T) {
+	t.Parallel()
+
+	_, err := New(Config{
+		ExportPath:    t.TempDir(),
+		ListenAddress: defaultListenAddress(defaultPort),
+	}, log.New(io.Discard, "", 0))
+	if err != nil {
+		t.Fatalf("new app: %v", err)
+	}
+}
+
+func TestNewRejectsRegistrationWithoutMachineID(t *testing.T) {
+	t.Parallel()
+
+	_, err := New(Config{
+		ExportPath:      t.TempDir(),
+		ListenAddress:   defaultListenAddress(defaultPort),
+		RegisterEnabled: true,
+		ControlPlaneURL: "http://127.0.0.1:8081",
+	}, log.New(io.Discard, "", 0))
+	if err == nil {
+		t.Fatal("expected missing machine id to fail")
+	}
+
+	if !strings.Contains(err.Error(), "BETTERNAS_NODE_MACHINE_ID") {
+		t.Fatalf("error = %q, want missing-machine-id message", err.Error())
+	}
+}