test: restore coverage removed in PR #8360 (#8779)

* test: restore maintenance mode coverage in TestVolumeMarkReadonlyWritableErrorPaths

PR #8360 removed the maintenance mode assertions because the refactored
check ordering (volume lookup before maintenance check) caused the
original test to hit "not found" instead of "maintenance mode" — the
test used a non-existent volume ID.

Restore coverage by allocating a real volume, then verifying:
- existing volume in maintenance mode returns "maintenance mode"
- non-existent volume in maintenance mode still returns "not found"
  (validating the new check ordering)

* test: add coverage for ScrubVolume MarkBrokenVolumesReadonly flag

PR #8360 added the mark_broken_volumes_readonly field to ScrubVolumeRequest
but no tests exercised the new logic paths. Add three integration tests:

- HealthyVolume: flag is a no-op when scrub finds no broken volumes
- CorruptVolume: corrupted .idx triggers broken detection; without the
  flag the volume stays writable, with the flag it becomes read-only
- MaintenanceMode: makeVolumeReadonly fails under maintenance and
  ScrubVolume propagates the error via errors.Join

* refactor: extract CorruptIndexFile and EnableMaintenanceMode test helpers

Move duplicated idx corruption and maintenance mode setup into
framework.CorruptIndexFile() and framework.EnableMaintenanceMode()
helpers. Use defer for file close in the corruption helper.

test/volume_server/framework/volume_fixture.go

@@ -5,6 +5,8 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"os"
+	"path/filepath"
 	"testing"
 	"time"
 
@@ -45,6 +47,39 @@ func UploadBytes(t testing.TB, client *http.Client, volumeURL, fid string, data
 	return DoRequest(t, client, req)
 }
 
+// CorruptIndexFile appends garbage bytes to a volume's .idx file on disk so
+// that CheckIndexFile detects a size mismatch during scrub.
+func CorruptIndexFile(t testing.TB, baseDir string, volumeID uint32) {
+	t.Helper()
+	idxPath := filepath.Join(baseDir, "volume", fmt.Sprintf("%d.idx", volumeID))
+	f, err := os.OpenFile(idxPath, os.O_WRONLY|os.O_APPEND, 0644)
+	if err != nil {
+		t.Fatalf("open idx file for corruption: %v", err)
+	}
+	defer f.Close()
+	if _, err := f.Write([]byte{0xDE, 0xAD}); err != nil {
+		t.Fatalf("corrupt idx file: %v", err)
+	}
+}
+
+// EnableMaintenanceMode puts the volume server into maintenance mode.
+func EnableMaintenanceMode(t testing.TB, ctx context.Context, client volume_server_pb.VolumeServerClient) {
+	t.Helper()
+	stateResp, err := client.GetState(ctx, &volume_server_pb.GetStateRequest{})
+	if err != nil {
+		t.Fatalf("GetState failed: %v", err)
+	}
+	_, err = client.SetState(ctx, &volume_server_pb.SetStateRequest{
+		State: &volume_server_pb.VolumeServerState{
+			Maintenance: true,
+			Version:     stateResp.GetState().GetVersion(),
+		},
+	})
+	if err != nil {
+		t.Fatalf("SetState maintenance=true failed: %v", err)
+	}
+}
+
 func ReadBytes(t testing.TB, client *http.Client, volumeURL, fid string) *http.Response {
 	t.Helper()