fix: tolerate re-import on an already-authorized pool session key

Session files persisted by earlier runs (where the import succeeded but
the login was never finalized client-side) hold a key that already
carries the account authorization; re-importing onto it is rejected by
the server with AUTH_BYTES_INVALID and the bootstrap aborted.

Treat AUTH_BYTES_INVALID on import as already-authorized: finalize the
login client-side with the user id returned by exportAuthorization and
let the workers verify the session by use, falling back to the
sequential download if it turns out to be broken.

Author: mateof

TelegramDownloader/Data/TelegramService.cs

@@ -350,8 +350,21 @@ private static bool ShouldUseMultiConnection(TL.Document document)
                         // With the UserId recorded, WTelegram's GetClientForDC
                         // handles the per-file-DC authorization automatically.
                         Auth_ExportedAuthorization exported = await client.Auth_ExportAuthorization(dc.id);
-                        Auth_AuthorizationBase auth = await pc.Auth_ImportAuthorization(exported.id, exported.bytes);
-                        pc.LoginAlreadyDone(auth);
+                        Auth_AuthorizationBase auth = null;
+                        try
+                        {
+                            auth = await pc.Auth_ImportAuthorization(exported.id, exported.bytes);
+                        }
+                        catch (RpcException rex) when (rex.Message == "AUTH_BYTES_INVALID")
+                        {
+                            // Session persisted by an earlier run whose import
+                            // succeeded but whose login was never finalized: the
+                            // key already holds the account authorization and
+                            // re-importing onto it is rejected. Record the login
+                            // client-side and let the workers verify by use.
+                            _logger.LogInformation("Download pool client {Index}: key already authorized on a previous run", index);
+                        }
+                        pc.LoginAlreadyDone(auth ?? new Auth_Authorization { user = new User { id = exported.id } });
                     }
                     _logger.LogInformation("Download pool client {Index} ready (homed on DC {Dc})", index, dc.id);
                     return pc;