fix(tdler): treat offset past EOF as end of file

gotd's downloader is size-unaware: when a file's size is an exact
multiple of the 1MiB part size, it issues one final upload.getFile
at offset == size, which Telegram rejects with 400 OFFSET_INVALID,
failing the whole download. Wrap the client to answer such requests
with an empty chunk, matching the EOF semantics the downloader
expects. Regression test uses a fake client with real server
behavior (OFFSET_INVALID past EOF).

common/tdler/dler.go

@@ -1,7 +1,10 @@
 package tdler
 
 import (
+	"context"
+
 	"github.com/gotd/td/telegram/downloader"
+	"github.com/gotd/td/tg"
 	"github.com/krau/SaveAny-Bot/common/utils/dlutil"
 	"github.com/krau/SaveAny-Bot/config"
 	"github.com/krau/SaveAny-Bot/pkg/consts/tglimit"
@@ -10,5 +13,23 @@ import (
 
 func NewDownloader(file tfile.TGFile) *downloader.Builder {
 	return downloader.NewDownloader().WithPartSize(tglimit.MaxPartSize).
-		Download(file.Dler(), file.Location()).WithThreads(dlutil.BestThreads(file.Size(), config.C().Threads))
+		Download(eofAwareClient{Client: file.Dler(), size: file.Size()}, file.Location()).
+		WithThreads(dlutil.BestThreads(file.Size(), config.C().Threads))
+}
+
+// eofAwareClient answers upload.getFile requests at or past the end of the
+// file with an empty chunk. gotd's downloader is size-unaware: for files
+// whose size is an exact multiple of the part size it issues one final
+// request at offset == size and expects an empty chunk, but Telegram rejects
+// it with 400 OFFSET_INVALID and the whole download fails.
+type eofAwareClient struct {
+	downloader.Client
+	size int64
+}
+
+func (c eofAwareClient) UploadGetFile(ctx context.Context, req *tg.UploadGetFileRequest) (tg.UploadFileClass, error) {
+	if req.Offset >= c.size {
+		return &tg.UploadFile{}, nil
+	}
+	return c.Client.UploadGetFile(ctx, req)
 }