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)
 }

common/tdler/dler_test.go

@@ -0,0 +1,112 @@
+package tdler
+
+import (
+	"bytes"
+	"context"
+	"sync"
+	"testing"
+
+	"github.com/gotd/td/tg"
+	"github.com/gotd/td/tgerr"
+
+	"github.com/krau/SaveAny-Bot/pkg/tfile"
+)
+
+// serverLikeClient mimics real Telegram upload.getFile behavior: it returns
+// up to limit bytes per chunk, and answers any offset at or past the end of
+// the file with 400 OFFSET_INVALID.
+type serverLikeClient struct {
+	data []byte
+
+	mu        sync.Mutex
+	maxOffset int64
+}
+
+func (c *serverLikeClient) UploadGetFile(_ context.Context, req *tg.UploadGetFileRequest) (tg.UploadFileClass, error) {
+	c.mu.Lock()
+	if req.Offset > c.maxOffset {
+		c.maxOffset = req.Offset
+	}
+	c.mu.Unlock()
+	if req.Offset >= int64(len(c.data)) {
+		return nil, tgerr.New(400, "OFFSET_INVALID")
+	}
+	end := min(len(c.data), int(req.Offset)+req.Limit)
+	return &tg.UploadFile{Bytes: c.data[req.Offset:end]}, nil
+}
+
+func (c *serverLikeClient) UploadGetFileHashes(context.Context, *tg.UploadGetFileHashesRequest) ([]tg.FileHash, error) {
+	return nil, nil
+}
+
+func (c *serverLikeClient) UploadReuploadCDNFile(context.Context, *tg.UploadReuploadCDNFileRequest) ([]tg.FileHash, error) {
+	return nil, nil
+}
+
+func (c *serverLikeClient) UploadGetCDNFileHashes(context.Context, *tg.UploadGetCDNFileHashesRequest) ([]tg.FileHash, error) {
+	return nil, nil
+}
+
+func (c *serverLikeClient) UploadGetWebFile(context.Context, *tg.UploadGetWebFileRequest) (*tg.UploadWebFile, error) {
+	return nil, nil
+}
+
+type memWriterAt struct {
+	b []byte
+}
+
+func (w *memWriterAt) WriteAt(p []byte, off int64) (int, error) {
+	copy(w.b[off:], p)
+	return len(p), nil
+}
+
+func TestDownloadServerLikeEOF(t *testing.T) {
+	const partSize = 1024 * 1024
+	tests := []struct {
+		name     string
+		size     int
+		parallel bool
+	}{
+		{"stream exact multiple of part size", 2 * partSize, false},
+		{"stream non-multiple", 2*partSize + 12345, false},
+		{"stream smaller than part size", 1234, false},
+		{"parallel exact multiple of part size", 2 * partSize, true},
+		{"parallel non-multiple", 2*partSize + 12345, true},
+		{"parallel smaller than part size", 1234, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			data := make([]byte, tt.size)
+			for i := range data {
+				data[i] = byte(i % 251)
+			}
+			client := &serverLikeClient{data: data}
+			file := tfile.NewTGFile(
+				&tg.InputDocumentFileLocation{ID: 1, AccessHash: 2},
+				client, int64(tt.size), "test.bin",
+			)
+
+			dl := NewDownloader(file)
+			var got []byte
+			var err error
+			if tt.parallel {
+				buf := make([]byte, tt.size)
+				_, err = dl.WithThreads(4).Parallel(context.Background(), &memWriterAt{b: buf})
+				got = buf
+			} else {
+				var buf bytes.Buffer
+				_, err = dl.Stream(context.Background(), &buf)
+				got = buf.Bytes()
+			}
+			if err != nil {
+				t.Fatalf("download failed: %v", err)
+			}
+			if !bytes.Equal(got, data) {
+				t.Fatalf("downloaded %d bytes, want %d matching bytes", len(got), len(data))
+			}
+			if client.maxOffset >= int64(tt.size) {
+				t.Fatalf("requested offset %d at or past EOF (size %d)", client.maxOffset, tt.size)
+			}
+		})
+	}
+}