Made forecast-period conditionCode optional

internal/normalizers/openmeteo/forecast_test.go

@@ -35,6 +35,9 @@ func TestBuildForecastUsesCanonicalTextDescription(t *testing.T) {
 	if got := run.Periods[0].TextDescription; got != expectedText {
 		t.Fatalf("TextDescription = %q, want %q", got, expectedText)
 	}
+	if run.Periods[0].ConditionCode == nil {
+		t.Fatalf("ConditionCode is nil, want mapped WMO code")
+	}
 
 	wantIssued := time.Date(2026, 3, 16, 19, 0, 0, 0, time.UTC)
 	if !run.IssuedAt.Equal(wantIssued) {