feat: support durable task cancellation without foreign keys

internal/storage/mysql/worker_results.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"time"
 
+	"git.yuntuo.hk/k2safe/orbit/internal/fault"
 	core "git.yuntuo.hk/k2safe/orbit/model"
 	"git.yuntuo.hk/k2safe/orbit/platform/auth"
 	pulse "git.yuntuo.hk/k2safe/orbit/pulse/model"
@@ -62,6 +63,9 @@ func (s *ExecutionStore) Finish(ctx context.Context, token auth.Token, b worker.
 			out = worker.CommandResult{Receipt: receipt, Lease: status, Deduplicated: true}
 			return nil
 		}
+		if in.Outcome == "CANCELLED" && task.view.CancelRequestedAt == nil {
+			return fault.Conflict
+		}
 		reason := ""
 		if a.ResultDigest != nil {
 			reason = "RESULT_CONFLICT"