fix: harden exclusive file creation and webdav writes

chfs-ng/internal/storage/multipath_test.go

@@ -46,3 +46,44 @@ func TestMultiRemoveCannotRemoveVirtualRoot(t *testing.T) {
 		t.Fatal("expected virtual root removal to fail")
 	}
 }
+
+func TestLocalCreateExclusiveRejectsExistingFile(t *testing.T) {
+	root := t.TempDir()
+	st := Local{Root: root}
+	if err := os.WriteFile(filepath.Join(root, "x.txt"), []byte("old"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	f, err := st.CreateExclusive("x.txt")
+	if !os.IsExist(err) || f != nil {
+		t.Fatalf("expected EEXIST, file=%v err=%v", f, err)
+	}
+	got, err := os.ReadFile(filepath.Join(root, "x.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "old" {
+		t.Fatalf("existing file changed: %q", got)
+	}
+}
+
+func TestLocalCreateExclusiveCreatesMissingFile(t *testing.T) {
+	st := Local{Root: t.TempDir()}
+	f, err := st.CreateExclusive("nested/x.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.WriteString("new"); err != nil {
+		f.Close()
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(filepath.Join(st.Root, "nested", "x.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "new" {
+		t.Fatalf("unexpected content: %q", got)
+	}
+}