Add tests for Square client hardening and GDPR deletion

Covers DeleteCustomer (success + NOT_FOUND no-op), doJSON truncation (oversized bodies + rune-safe capBody), validCardID rejection without leaking the token, paymentFromSquare empty-card consistency, and mock-side customer-ID redaction in logs.

backend/internal/square/square_dev_test.go

@@ -797,6 +797,58 @@ func TestDevClient_CreateCustomer_EmptyEmail(t *testing.T) {
 	assert.Contains(t, err.Error(), "email")
 }
 
+func TestDevClient_DeleteCustomer(t *testing.T) {
+	client := NewDevClient().(*MockClient)
+	ctx := context.Background()
+
+	cust, err := client.CreateCustomer(ctx, "Jane Doe", "jane@example.com")
+	require.NoError(t, err)
+
+	err = client.DeleteCustomer(ctx, cust.ID)
+	require.NoError(t, err)
+
+	client.mu.RLock()
+	defer client.mu.RUnlock()
+	assert.Len(t, client.customers, 0, "deleted customer must be removed from the mock store")
+}
+
+func TestDevClient_DeleteCustomer_NotFoundIsNoop(t *testing.T) {
+	client := NewDevClient().(*MockClient)
+	ctx := context.Background()
+
+	// Deleting a customer the mock never created mirrors Square's NOT_FOUND —
+	// idempotent re-deletion must return nil (GDPR re-runs are safe).
+	err := client.DeleteCustomer(ctx, "cus_missing")
+	require.NoError(t, err)
+}
+
+// TestDevClient_CustomerID_RedactedInLogs verifies the mock never logs a full
+// customer ID (S-2 convention): DeleteCustomer's entry/success lines and
+// CreateCustomer's dedup-hit/created lines all use the tokenPrefix redaction,
+// mirroring how prod redacts ccof: tokens.
+func TestDevClient_CustomerID_RedactedInLogs(t *testing.T) {
+	client := NewDevClient().(*MockClient)
+	ctx := context.Background()
+
+	cust, err := client.CreateCustomer(ctx, "Jane Doe", "jane@example.com")
+	require.NoError(t, err)
+
+	var buf bytes.Buffer
+	log.SetOutput(&buf)
+	defer log.SetOutput(os.Stderr)
+
+	err = client.DeleteCustomer(ctx, cust.ID)
+	require.NoError(t, err)
+
+	logs := buf.String()
+	if strings.Contains(logs, cust.ID) {
+		t.Errorf("full customer id %q leaked into mock logs: %q", cust.ID, logs)
+	}
+	if !strings.Contains(logs, tokenPrefix(cust.ID)) {
+		t.Errorf("expected redacted customer id %q in logs, got %q", tokenPrefix(cust.ID), logs)
+	}
+}
+
 func TestDevClient_CancelCheckout_CancelsPending(t *testing.T) {
 	client := NewDevClient().(*MockClient)
 	client.HoldCheckouts = true