feat: restore credit limits and harden SMS sending

gateway/internal/upstream/deliver_test.go

@@ -86,3 +86,14 @@ func TestHandleCMPP2DeliverReceiptPostsReceiptEvent(t *testing.T) {
 		t.Fatal("timed out waiting for receipt event")
 	}
 }
+
+func TestReceiptStatusTreatsNonDeliveredFinalStatesAsUndelivered(t *testing.T) {
+	for _, stat := range []string{"UNKNOWN", "UNDELIV", "EXPIRED", "DELETED", "REJECTD"} {
+		if got := receiptStatus(stat); got != "undelivered" {
+			t.Fatalf("receiptStatus(%q) = %q, want undelivered", stat, got)
+		}
+	}
+	if got := receiptStatus(""); got != "unknown" {
+		t.Fatalf("receiptStatus(empty) = %q, want unknown", got)
+	}
+}