fix(batch): leave no record for a canceled job nobody owns after discovery

Canceling a job ID this node does not own recorded a canceled owner so
that startup discovery, which may still admit the manifest of that ID,
would not run a job that was canceled before the node had loaded it.
The record was kept for any ID, at any time, and never removed: a caller
with admin:CancelBatchJob could grow the ownership table by one entry per
cancellation of an unknown ID, and the retry scheduler walks that table
every second.

Once discovery has finished, nothing admits a job this node does not own:
new submissions use new IDs, so the record has nothing left to fence.
canceler now records a canceled owner for an unknown ID only while
discovery is still running. Canceling an owned job, and the fencing of a
cancellation during discovery, are unchanged, and the cancel handler
still removes the manifest.

TestBatchCancelUnknownJobAfterDiscovery cancels an unknown ID during
discovery and requires its admission to be refused, then cancels a
thousand unknown IDs after discovery and requires the ownership table
not to grow. Before this change it held a thousand records.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/batch-handlers.go

@@ -2520,15 +2520,21 @@ func (j *BatchJobPool) canceler(jobID string, cancel bool) error {
 	if !cancel {
 		return nil
 	} // Worker completion uses its generation token.
-	if j.jobOwners == nil {
-		j.jobOwners = make(map[string]*batchJobOwner)
-	}
 	if owner := j.jobOwners[jobID]; owner != nil {
 		if owner.cancel != nil {
 			owner.cancel()
 		}
 		j.jobOwners[jobID] = &batchJobOwner{state: "canceled"}
-	} else {
+		return nil
+	}
+	// Only startup discovery admits a manifest this node does not own yet.
+	// A cancellation before that admission leaves a record that fences it;
+	// once discovery has finished, nothing admits the ID again (submissions
+	// use new IDs), so an unknown ID leaves no record to accumulate.
+	if !j.discoveryDone {
+		if j.jobOwners == nil {
+			j.jobOwners = make(map[string]*batchJobOwner)
+		}
 		j.jobOwners[jobID] = &batchJobOwner{state: "canceled"}
 	}
 	return nil