fix(scanner): add nil guards to cursor wrapping (#5139)

* fix(persistence): add nil guards to cursor wrapping in folder and mediafile repos

Prevent SIGSEGV panic when queryWithStableResults yields a zero-value
struct on the rows.Err() path (e.g., "database is locked" during
concurrent scanning). Extract cursor wrapping into wrapFolderCursor and
wrapMediaFileCursor with nil checks matching the existing pattern in
album_repository.go.

Fixes #5138

* fix(persistence): wrap original cursor error in nil guard messages

Use %w to preserve the underlying error (e.g., "database is locked")
so callers can use errors.Is/As for root cause analysis. Tests now
verify the original error is accessible via errors.Is.

* fix(persistence): add nil guards and error wrapping in album, folder, and mediafile cursor functions

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

persistence/mediafile_repository.go

@@ -3,6 +3,7 @@ package persistence
 import (
 	"context"
 	"fmt"
+	"iter"
 	"slices"
 	"strconv"
 	"strings"
@@ -231,17 +232,7 @@ func (r *mediaFileRepository) GetCursor(options ...model.QueryOptions) (model.Me
 	if err != nil {
 		return nil, err
 	}
-	return func(yield func(model.MediaFile, error) bool) {
-		for m, err := range cursor {
-			if m.MediaFile == nil {
-				yield(model.MediaFile{}, fmt.Errorf("unexpected nil mediafile: %v", m))
-				return
-			}
-			if !yield(*m.MediaFile, err) || err != nil {
-				return
-			}
-		}
-	}, nil
+	return wrapMediaFileCursor(cursor), nil
 }
 
 // FindByPaths finds media files by their paths.
@@ -371,13 +362,21 @@ func (r *mediaFileRepository) GetMissingAndMatching(libId int) (model.MediaFileC
 	if err != nil {
 		return nil, err
 	}
+	return wrapMediaFileCursor(cursor), nil
+}
+
+func wrapMediaFileCursor(cursor iter.Seq2[dbMediaFile, error]) model.MediaFileCursor {
 	return func(yield func(model.MediaFile, error) bool) {
 		for m, err := range cursor {
+			if m.MediaFile == nil {
+				yield(model.MediaFile{}, fmt.Errorf("unexpected nil mediafile (%v): %w", m, err))
+				return
+			}
 			if !yield(*m.MediaFile, err) || err != nil {
 				return
 			}
 		}
-	}, nil
+	}
 }
 
 // FindRecentFilesByMBZTrackID finds recently added files by MusicBrainz Track ID in other libraries