fix(actions): run TransferLogs on UpdateLog{Rows:[], NoMore:true} (#37631) (#37687)

Backport #37631 by @silverwind

`UpdateLog` short-circuits on `len(Rows)==0` before honoring `NoMore`,
so a final empty `UpdateLog{NoMore:true}` never runs `TransferLogs`. The
task's `dbfs_data` rows are then never moved to log storage and never
deleted.

The bug has been latent since the original Actions implementation,
`act_runner` versions after
[runner#819](https://gitea.com/gitea/runner/pulls/819) trip it
deterministically.

Fix: let `NoMore=true` with no new rows fall through to `TransferLogs`.
Bail when the runner has outrun the server (`Index > ack`) even with
`NoMore`, since archiving a log with a gap is worse than retrying.
Always call `WriteLogs` so `offset==0` bootstraps an empty DBFS file in
the no-output case (otherwise `TransferLogs` would fail at `dbfs.Open`).

Fixes: https://github.com/go-gitea/gitea/issues/37623
Ref: [runner#952](https://gitea.com/gitea/runner/pulls/952)
Ref: [runner#950](https://gitea.com/gitea/runner/pulls/950)

---
This PR was written with the help of Claude Opus 4.7

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

routers/api/actions/runner/runner.go

@@ -264,7 +264,16 @@ func (s *Service) UpdateLog(
 	}
 	ack := task.LogLength
 
-	if len(req.Msg.Rows) == 0 || req.Msg.Index > ack || int64(len(req.Msg.Rows))+req.Msg.Index <= ack {
+	// Trim rows the runner already had acked.
+	var rows []*runnerv1.LogRow
+	if req.Msg.Index <= ack && int64(len(req.Msg.Rows))+req.Msg.Index > ack {
+		rows = req.Msg.Rows[ack-req.Msg.Index:]
+	}
+
+	// Bail unless we have new rows or a NoMore to finalize. Even with
+	// NoMore, bail when the runner has outrun the server — archiving a
+	// log with a gap is worse than asking it to retry.
+	if len(rows) == 0 && (!req.Msg.NoMore || req.Msg.Index > ack) {
 		res.Msg.AckIndex = ack
 		return res, nil
 	}
@@ -273,7 +282,9 @@ func (s *Service) UpdateLog(
 		return nil, status.Errorf(codes.AlreadyExists, "log file has been archived")
 	}
 
-	rows := req.Msg.Rows[ack-req.Msg.Index:]
+	// WriteLogs is called even with no rows: with offset==0 it bootstraps
+	// an empty DBFS file so TransferLogs below has something to read when
+	// the runner finalizes a task that produced no log output.
 	ns, err := actions.WriteLogs(ctx, task.LogFilename, task.LogSize, rows)
 	if err != nil {
 		return nil, status.Errorf(codes.Internal, "unable to append logs to dbfs file: %v", err)