fix(s3): return NoSuchKey for missing copy sources

- Translate missing source errors during both CopyObject reads
- Add regression coverage for copying a nonexistent source key

Co-authored-by: Codex <267193182+codex@users.noreply.github.com>
Signed-off-by: jyxjjj <16695261+jyxjjj@users.noreply.github.com>

server/s3/backend.go

@@ -451,11 +451,17 @@ func (b *s3Backend) CopyObject(ctx context.Context, srcBucket, srcKey, dstBucket
 	fmeta, _ := op.GetNearestMeta(srcFp)
 	srcNode, err := fs.Get(context.WithValue(ctx, conf.MetaKey, fmeta), srcFp, &fs.GetArgs{})
 	if err != nil {
+		if errs.IsObjectNotFound(err) {
+			return result, gofakes3.KeyNotFound(srcKey)
+		}
 		return result, err
 	}
 
 	c, err := b.GetObject(ctx, srcBucket, srcKey, nil)
 	if err != nil {
+		if errs.IsObjectNotFound(err) {
+			return result, gofakes3.KeyNotFound(srcKey)
+		}
 		return
 	}
 	defer func() {

server/s3/copy_test.go

@@ -0,0 +1,17 @@
+package s3
+
+import (
+	"context"
+	"testing"
+
+	"github.com/OpenListTeam/gofakes3"
+)
+
+func TestCopyObjectMissingSourceReturnsNoSuchKey(t *testing.T) {
+	b, _ := setupMultipartBackend(t)
+
+	_, err := b.CopyObject(context.Background(), "mp", "missing.txt", "mp", "copy.txt", nil)
+	if code := s3ErrorCode(err); code != gofakes3.ErrNoSuchKey {
+		t.Fatalf("CopyObject() error = %v, want NoSuchKey", code)
+	}
+}