Block delete for in-progress access review campaigns

Reject deletion while a campaign is fetching sources so workers are
not racing a removed record. All other statuses remain deletable.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/accessreview/errors_test.go

@@ -57,6 +57,15 @@ func TestCampaignClientErrors(t *testing.T) {
 			wantText: fmt.Sprintf("access review campaign %q is not in draft", campaignID),
 			sentinel: accessreview.ErrCampaignNotDraft,
 		},
+		{
+			name: "not deletable",
+			err:  accessreview.NewCampaignNotDeletableError(campaignID),
+			wantText: fmt.Sprintf(
+				"access review campaign %q cannot be deleted while it is in progress",
+				campaignID,
+			),
+			sentinel: accessreview.ErrCampaignNotDeletable,
+		},
 		{
 			name: "not pending actions",
 			err:  accessreview.NewCampaignNotPendingActionsError(campaignID),